CAMPUS is where the university and all other university-related buildings are. Everything is on campus: all faculty buildings and departments, libraries, lecture halls, gyms and stadiums, medical facilities, food courts, restaurants, coffee shops, student dorms and parks. Everything in connection to university and student life goes on on campus.
University campuses in USA of course differ from each other, mostly due to the size of each university. Basically, there are three university sizes: smaller, middle and big universities. Some of them only have around 100 undergraduate students enrolled while the biggest school between 40 and 50 thousand students per year (for example, University of Florida had 52,532 students enrolled in the year 2014/2015).
During their studies, students can live either on or off campus (in a private flat/house). International students are advised to live on campus, at least for the first year, so they can meet more students and get used to living in a different culture. Universities offer all sorts of activities. You can choose among all sorts of clubs, and become a part of a group that has the same interests and hobbies as you do. This will make you feel more at home, relaxed and safe in a new environment.
Students really have the best facilities for studying, sports and leisure time. The conditions for successfully studying truly are one of the best ones in the world. A big part of that are also professors, who encourage their students every step of the way, appreciate all the extra work and are available almost 24 hours a day (e-mail or telephone).