Hostels can range from a room with 2 or 3 beds to a room with 20 beds. You may not be guaranteed to have the room be exclusive to your reservation, so they often have lockers or drawers that you can "rent" locks for to keep your valuables in.
I stayed in about 30 hostels during my two month backpacking trip in Europe last year. Some are much nicer than others... and sometimes spending the extra 15 euros per night would get my buds and I a private room instead of a 20 bed dormitory room!
Hostelworld.com is the best site for hostels and has a lot of information and REVIEWS which are key!