Source : https://www.virtualcuriosities.com/articles/1385/overview-of-toolbox-in-krita
All other images are sourced from the Krita website.



Here is a short explanation for each tool (if needed).
Clicking on a tool name brings you to the official documentation page if you want details on shortcuts or something else.
There is also a demonstration of every tool in a youtube video at the end of the Toolbox section.

Shape Selection Tool[docs.krita.org] : Only for vector layers. Select, transform, and access a vector shape’s options. There are a lot of different properties and things you can do with each vector shape

Shape Edit Tool[docs.krita.org] : Only for vector layers. Edit vector shapes by moving the nodes that change their path.

Text Tool[docs.krita.org]

Calligraphy Tool[docs.krita.org] : Create vector calligraphy

Freehand Brush Tool[docs.krita.org] : Your most used tool, paint on layers with no constraints. To switch the brush, use the Brush Preset Docker. Control switches to color sampler (left/right click to sample fore/background).

Straight Line Tool[docs.krita.org] : can be used with a brush

Rectangle Tool[docs.krita.org] : can be used with a brush, on a vector layer...

Ellipse Tool[docs.krita.org] : can be used with a brush

Polygon Tool[docs.krita.org] : Draw polygons. Double-click to connect the last vertex to the starting point.

Polyline Tool[docs.krita.org] : Similar to Polygon tool, only difference is the double-click doesn't make the last vertex connect to the starting point.

Bezier Curve Tool[docs.krita.org] : Draw curvesYou can draw curves by using this tool. Left-click to indicate the starting point of the curve, then click again for consecutive control points of the curve. While creating a control point, drag to create the handles, they will show as red lines.With an intermediate control point (i.e. a point that is not the starting point and not the ending point), you can move the direction handles separately to have the curve enter and leave the point in different directions. After editing a point, you can just click on the canvas to continue adding points to the curve. When creating a path on a vector layer, the resulting path can be further edited with the Shape Edit Tool.



The path preview that shows while drawing a path. The start of the path is indicated with a white square, the preview for the path itself in black, and the red dotted lines being the control points for the current handle.

Double left-click on any point of the curve or press the Enter key to finish drawing, or press the Esc key to cancel the entire curve.While drawing a curve, pressing the Ctrl key while dragging will push the handles both ways. The Alt key will create a sharp corner, and the Shift key will allow you to make a handle while at the end of the curve. Right-click will undo the last added point.

Freehand Path Tool[docs.krita.org] : Make shapes filled with a color or pattern, or outlined with a brush. Particularly good for laying large swaths of color quickly.

Dynamic Brush Tool[docs.krita.org] : Custom smoothing dynamics to your brush. This will give you similar smoothing results as the normal freehand brush. Recommended values around 0.02 for Mass, 0.92 for Drag. Mass = average your movements to make it smoother. Higher makes brush slower. Drag = rubber band effect that will help your lines come back to the cursor. Lower values make effect more extreme.

Multibrush Tool[docs.krita.org] : Draw using multiple instances of a freehand brush stroke at once. Very fun with 5 available modes in the options : Symmetry, Mirror, Translate, Snowflake & Copy Translate.Check

Crop Tool[docs.krita.org] : Crop an image or layer, reducing it to the the content within the area selecte.

Move Tool[docs.krita.org]

Transform Tool[docs.krita.org] : lets you quickly transform the current selection or layer. Basic transformation options include resize, rotate and skew. In addition, in the option docker there are advanced transforms such as Free Transform, Perspective, Warp, Cage and Liquify.

Fill Tool[docs.krita.org] : Clicking anywhere on screen will allow that area to be filed with the foreground color.The options found in the Tool Options docker and outlined below will give you a great deal of flexibility working with layers and selections.

Enclose and Fill Tool[docs.krita.org] : A different method of filling large colored areas. Instead of selecting each area you want to fill, or drawing the associated color of each area, you select an area, and it will fill everything inside said area.

Simple usage of the enclose and fill. Draw a rectangle around everything you wish to fill and Krita will try to find all possible areas inside that rectangle that can be filled.

Gradient Tool[docs.krita.org] : Left-Click dragging this tool over the active portion of the canvas will draw out the current gradient. If there is an active selection then, similar to the Fill Tool, the paint action will be confined to the selection’s borders.

Color Sampler Tool[docs.krita.org] : Choose a point from the canvas and make the color of that point the active foreground color. When a painting or drawing tool is selected the Color Sampler tool can also be quickly accessed by pressing the Ctrl key.

Colorize Mask[docs.krita.org] : A tool for quickly coloring line art, the Colorize Mask Editing tool can be found next to the gradient tool on your toolbox. Creates a colorize mask layer. Démo

Smart Patch Tool[docs.krita.org] : Seamlessly remove elements from the image. It does this by letting you draw the area which has the element you wish to remove, and then it will attempt to use patterns already existing in the image to fill the blank. You can see it as a smarter version of the clone brush.
Accuracy : balance your level of accuracy. Low accuracy will be faster but less precise. High accuracy takes more computing time for better results.
Patch size : how big the size of the pattern to choose is. If the surrounding image has mostly small elements, like branches, a small patch size will give better results, while a big patch size will be better for images with big elements, so they get reused as a whole.

Assistant Tool[docs.krita.org] : Create, edit, and remove drawing assistants on the canvas, for drawing perspective. The tool options gives you different types of assistants, add new assistants, and to save/load assistants.

Reference Images Tool[docs.krita.org] : The reference images tool is a replacement for the reference images docker. You can use it to load images from your disk as reference, which can then be moved around freely on the canvas and placed wherever.

Measure Tool[docs.krita.org] : Measure distances and angles

Some common mecanics between all Selection tools : Link[docs.krita.org]
Rectangular Selection Tool[docs.krita.org]

Elliptical Selection Tool[docs.krita.org]

Freehand Selection Tool (lasso)[docs.krita.org]

Polygonal Selection Tool[docs.krita.org]

Contiguous Selection Tool (magic wand)[docs.krita.org] : Make Selections by selecting a point of color. It will select any contiguous areas of a similar color to the one you selected. You can adjust the “fuzziness” of the tool, a lower number will select colors closer to the color that you chose in the first place.

Path Selection Tool (bezier)[docs.krita.org] : Make a Selections of an area by drawing a path around it. Click where you want each point of the path to be. Click and drag to curve the line between points. Finally click on the first point to close your path.

Similar Color Selection Tool (color picker)[docs.krita.org] : Make Selections by selecting a point of color. It will select any areas of a similar color to the one you selected. You can adjust the “fuzziness” of the tool, a lower number will select colors closer to the color that you chose in the first place.

Magnetic Selection Tool[docs.krita.org] : Make freeform Selections, but unlike the Polygonal Selection Tool or the Freehand Selection Tool, it will try to magnetically snap to sharp contrasts in your image, simplifying the creation of selection drastically.

Zoom Tool[docs.krita.org]

Pan Tool[docs.krita.org]

Layers reference page[docs.krita.org], how to use the Layers docker, and shortcuts.
Layers and Masks reference page[docs.krita.org], general info on their use.

Clone Layers[docs.krita.org]
This layer is a live copy of another layer. You can’t draw on it directly, but you can apply effects and masks to it, which will reflect on the clone layer in real-time.
A clone layer is a layer that keeps an up-to-date copy of another layer. You cannot draw or paint on it directly, but it can be used to create effects by applying different types of layers and masks (e.g. filter layers or masks).

For example, if you were painting a picture of some magic person and wanted to create a glow around them that was updated as you updated your character, you could:
Have a Paint Layer where you draw your character, then use the Clone Layer feature to create a clone of the layer that you drew your character on. Apply an HSV filter mask to the clone layer to make the shapes on it white (or blue, or green etc.) and finally apply a blur filter mask to the clone layer so it looks like a “glow”.
As you keep painting and adding details, erasing on the first layer, Krita will automatically update the clone layer, making your “glow” apply to every change you make.

File Layers[docs.krita.org]
Transform an image file from your computer into a layer in Krita, useful for collaboration and referencing.
File Layers are references to files outside of the document: If the referenced document updates, the file layer will update. Do not remove the original file on your computer once you add it to Krita, it will break the file layer. If Krita cannot find the original file, it’ll ask you where to find it. File layers cannot display animations.
File Layers can currently not be painted on. If you want to transform a file layer, you need to apply a transformation mask to it and use that.

Fill Layers[docs.krita.org]
A Fill Layer is a special layer that Krita generates on-the-fly that can contain either a pattern or a solid color.
A fill layer is a single-channel layer, meaning it only has transparency. Therefore, you can erase and paint on fill-layers to make them semi-opaque, or for when you want to have a particular color only. Being single channel, fill layers are also a little bit less memory-consuming than regular 4-channel paint layers.

Filter Layer[docs.krita.org]
Filter layers show whatever layers are underneath them, but with a filter such as Layer Styles, Blur, Levels, Brightness / Contrast. For example, if you add a Filter Layer, and choose the Blur filter, you will see every layer under your filter layer blurred.
Unlike applying a filter directly on to a section of a Paint Layer, Filter Layers do not actually alter the original image in the Paint Layers below them. Once again, non-destructive editing! You can tweak the filter at any time, and the changes can always be altered or removed.
Unlike Filter Masks though, Filter Layers apply to the entire canvas for the layers beneath. If you wish to apply a filter layer to only some layers, then you can utilize the Group Layer feature and add those layers into a group with the filter layer on top of the stack.

Filter Masks[docs.krita.org]
Filter masks show an area of their layer with a filter (such as blur, levels, brightness / contrast etc.). For example, if you select an area of a paint layer and add a Filter Mask, you will be asked to choose a filter. If you choose the blur filter, you will see the area you selected blurred.
Unlike applying a filter to a section of a paint layer directly, filter masks do not permanently alter the original image. This means you can tweak the filter (or the area it applies to) at any time. Changes can always be altered or removed.
Unlike filter layers, filter masks apply only to the area you have selected (the mask).

Group Layers[docs.krita.org]
The created layer will be grouped with the currently selected layer. Helps in organizing multiple layers together, simplifying the workflow.
Group layers allow you to segregate some layers so you can hide these quickly, or so you can recursively transform the content of the group, or so you can apply a mask to all the layers inside this group as if they are one (e.g. by dragging an existing mask to a group layer), etc.. You can quickly create a group layer by selecting the layers that should be grouped together and then pressing the Ctrl + G shortcut.

Layer Styles[docs.krita.org]
Layer styles are effects that are added on top of your layer. They are editable and can easily be toggled on and off. To add a layer style to a layer go to Layer ‣ Layer Style. You can also right-click a layer to access the layer styles.
When you have the layer styles window up, make sure that the Enable Effects item is checked.
There are a variety of effects and styles you can apply to a layer. When you add a style, your layer docker will show an extra “Fx” icon. This allows you to toggle the layer style effects on and off.

Paint Layers[docs.krita.org]
It’s a standard layer where you can paint or draw as you would on the main canvas.

Selection Masks[docs.krita.org]
Local Selection masks let you remember and recall edit a selection on a layer. They work in a similar way to extra channels in other image editing programs. One difference is Krita's ability to assign them to specific layers and activate a selection with a single click on the layer. Just click the round icon with the dotted outline on the local selection layer in the Layers docker.
To create a Local Selection Mask, you must first create a selection, then right-click on the desired layer and select Local Selection.
When isolating a selection mask with the Alt + left-click shortcut, you can perform transformation, deformation and paint operations on the selection layer, modifying the selection.A single layer can contain multiple Local Selection Masks. Repeating.
A single layer can contain multiple Local Selection Masks (LSM). This is important because it means that you can, for instance, have several different outline parts of an image and save each as its own LSM and then recall it with a single click. Without using LSM you would have to create layer upon layer for each mask. Not only would this be inefficient for you but also for Krita and the program would slow down trying to keep up with it all. LSM’s are one of the most important features in Krita!

You can modify the global selection the same way you can with a local-selection. To do so, you first need to activate the global selection as a layer node. To do so, go into Select ‣ Show Global Selection Mask. The global selection, if you have anything selected, will now appear on the top of the layer stack as a selection mask.

Split Alpha[docs.krita.org]
Sometimes especially in the field of game development, artists need to work with the alpha channel of the texture separately. To assist such workflow, Krita has a special functionality called Split Alpha. It allows splitting alpha channel of a paint layer into a separate Transparency Masks. The artist can work on the transparency mask in an isolated environment and merge it back when he has finished working.

Transform Masks[docs.krita.org]
Rather than working with a brush to affect the mask, transform masks allow you to transform (move, rotate, shear, scale and perspective) a layer without applying the transform directly to the paint layer and making it permanent.
In the same way that Filter and Transparency Masks can be attached to a Paint layer and are non-destructive, so too can the Transform Mask.

Transparency Masks[docs.krita.org]
The Transparency mask allows you to selectively show or hide parts of a layer. By using a mask, you are able to avoid deleting parts of an image that you just might want in the future. This allows you to work non-destructively.
In addition, it allows you to do things like remove a portion of a layer in the layer stack so you can see what’s behind it. One example would be if you wanted to replace a sky, but were unsure of how much you wanted to replace.
You can always fine-tune and edit what you want visible and any layer. If you discover you’ve hidden part of your paint layer accidentally, you can always show it again just by painting white on your transparency mask.
This makes for a workflow that is extremely flexible and tolerant of mistakes.

Vector Layers[docs.krita.org]
Use this layer to add vector graphics. It’s designed exclusively for vector elements.
Ideal for non-destructive, scalable graphic elements.

BONUS : alpha inheritance notion