In American English, a hostel is a place where travelers, usually tourists, can stay overnight. It's cheaper than a hotel. You might sleep in your own room or in a bigger room with strangers. Many hostels are just for young people, "youth hostels," since they usually can't afford regular hotel rooms. A dormitory is a building on the campuses of colleges and universities where the students live. Usually there are many rooms and two students share one room. There are common bathrooms (one or two on each floor) and sometimes a TV room or other common area. Outside the US I have heard dormitory to mean a big room with a lot of beds like you find in an orphanage, hospital, etc.