World of Warcraft: Level ??? D:?

Okay, so this is probably a really noob question, but in World of Warcraft I keep seeing played characters at deadly boss levels. So, I see them and think they MUST be pretty awesome players, but I check their gear and some of them are using low level stuff. I guess I'm wondering why and how they are at that level and why there are so many of them running around terrorizing starter areas. XD Thanks!