As someone not familiar with US universities and fraternities (outside of popular college movies), I have to ask: Is it mandatory for students to join these fr

As someone not familiar with US universities and fraternities (outside of popular college movies), I have to ask: Is it mandatory for students to join these fraternities? And if so, why? I know very little about the subject, as I was never interested in this, but I think here in Austria we have these kinds of fraternities as well, but they are very rare and don't seem particularly interesting. Or maybe that's just me.