I knew 4chan was not for me when I saw a picture of two teenage boys having sex. Jesus christ.
Bob: Yeah, that sure sounds like 4chan.org to me
Person 2: Oh yes, I did. She accidentally the whole thing!
Whoever wrote the first definition must have been on crack.
The most famous board is certainly /b/, the random board. After that probably comes /a/, the anime board, and then maybe /v/, the video games board.
...See Encyclopedia Dramatica for where much of 4chan goes to die.