Hostels come in all shapes and sizes nowadays.
City hostels tend to lean more towards the social/fun aspect of city life, offering information about pubs, tours of the city, thrift shops and night clubs. They can be much noisier than country hostels, since they often have no curfew and are open 24 hours a day. The bigger the hostel, and the more night-life oriented the city it's in, the more social life you will find.
Country hostels are a very different thing. They're mostly frequented by people who are travelling through the country, possibly walking or hiking. This means people wake up earlier, and go to sleep earlier, and usually like it quiet!