They are different from hotels (where you have a room to yourself with own amenities, despite it costing more) while hostels are the opposite: a room consisting of bunk beds as it's a shared quarters with backpackers (no private bathroom, amenities are communal) but it's cheaper than staying at a hotel.

Experiences vary depending on country and type of hostel. The thing that also sets them apart from hotels are that: the mattresses used at hostels are "second hand" (hotels give the mattresses they deem bad discarding them to hostels for "charity" aka. handouts) while hotels get newer or better mattresses.

You also lack privacy at hostels (as it's a shared space) while hotels respect privacy at least. Also they say it's ill advised on leaving belongings unattended at hostels since it's a room shared with strangers (you don't know their intentions nor trust them) as there are horror stories of people having their stuff stolen.

Room Service isn't even a thing at hostels, as it's a "go get it yourself" or "order food" type environment, a communal kitchen where appliances exist (electric kettle, microwave) with the table being also a shared space. That's not the case with hotels as they have room service dedicated to bringing in food.