A bunch of guys in the same fraternity called "fraternity brothers" getting together and having an awesome time. Generally, there are more girls than guys because girls are attracted to strong male organizations. Guys who are not in these fraternities usually look in from the outside with jealousy because there girlfriends are in there.
GDI 1: "Hey man, did you see that frat party last night?"
GDI 2: "Yeah man, that looked awesome! SAE sure does throw some awesome parties! I wish I was one..."