Typically speaking, you're talking about two different subjects. That is, frat LIFE, and frat INITIATIONS. They're very different things.
The initiation itself, is like most secret societies. It's a symbolic ritual, oaths, pledges, etc. In mine, I was shirtless, but my pants were never removed. You have to remember, that most of the modern fraternities were founded in the very uptight 1800s, and many have semi-religious bases to them.
Now, leading up to initiation is "Hell Week". All kinds of strange things go on, and it varies from house to house, and campus to campus, and is a mix of national and local traditions. Most of it boils down to absurd tasks, and sleep deprivation.
Once you're an active member, frat life... Well, there are NO set rules there. While there are certain rules and codes that houses follow, the rest of it is like any other all-guy group thrown together. Some share girls, some would never think about it. Some do circle jerks, some are homophobic.
Finally, if you do a search, there was already a frightenly similar thread started by Ganymede, a while back.