The curfew factor may be important to you if you want to stay out late, but in a city like Venice, Italy, the entire town goes to sleep early so the only reason to stay up late is talking to other travelers in the hostel. In Barcelona on the other hand, the city never sleeps, so you might not come in until 5 a.m., but the hostel also lets you sleep late. Hostels do not normally provide linens, but you can pay a bit extra to rent a clean stack of sheets and pillow case from them. Most people bring their own sleepsack which is a sheet folded in half and sewn up like a sleeping bag. Sometimes a sleepsack includes a pillow case at the top which is also sewn on.
Hostels do provide a bed, blanket and pillow.
Other extras offered by hostels will vary, but include things like laundry facilities, travel libraries, lockers, vending machines or stores, bicycle rentals and in certain areas, ski rentals and storage or swimming pools and hot tubs. The greatest thing about hostels is that they offer a friendly accomodation for budget travelers of all ages. It's a place to spend the night and meet people from all over the world who share your sense of adventure and love of travel. A place to talk to fellow travelers to get the scoop on places they have been because the difference between a boring, dirty place and a charming, clean place with a little atmosphere and lots of excited travelers can be what makes or breaks a city. Check out the room first before you commit to staying there and don't be afraid to leave if something is not to your liking.