This will be sort of a broad question considering i dont know much about photo agencies. Could someone explain to me how they work, and why photographers use them. I hear about it all the time but don't know any information. . . And also if anybody knows any good ones. . . Thanks!

A great book to grab is Successful Self-Promotion for Photographers by Elyse Weissberg. It talks about how they work, how to acquire an agent  and gives great advice on self-promotion

A photo rep gets the photographer work in exchange for a ghastly high percentage.  Typically you buy several complete portfolios for the rep, who then sends them out all over trying to stir up assignments.  Reps are not exclusive, so work you get yourself, or through another path, doesn't get hit for the rep's fee.

As with everything else there are strata of reps from those who handle the big boys to those who are bottom feeders.  It isn't a case of how much you pay them, but how much work they get you.  As such, the top reps are very exclusive about who they will take, and like top model agencies, won't rep someone who won't bring in the bux.

-Don