The game consists of 50 levels; however the last level is not available in practice mode (reference). The objective of each is to steer the green cube to the exit (blue cube). Avoid coming in contact with any other cubes as they will kill you. There are also 9 collectable diamond challenges within some of the levels even though the status bar indicates 10 (Reference).
I was only able to locate 6 of them on levels 16, 22, 26, 29, 30, and 36.

The last checkpoint is stored in the registry: HKCU\Software\DefaultCompany\Why So Evil
So if you really want to play level 50 on practice mode then start a new game and quit the game on the first level, then change the value of the "LastPracticeLevel_h2818924898" to 50 using regedit. Or, Just copy this code into Notepad and save it with a .reg extension. Simply double-click the file to load the save game into your registry. Be sure to backup your current key if you don’t want to lose it.
Now, after opening the game, choose to "Continue from last" and you will be taken to level 50 ... good luck.