How do you start a raid in World of Warcraft and what are they actually for?

I want to start a raid in world of Warcraft but I dont really understand how to do it and what the benefits of doing it are. Ive pretty much just played the game by myself and never really done many of the dungeons or anything like that. I have a level 65 warrior if that helps at all. If anyone can tell me how to do it I would appreciate it.