Start right here:
www.boxrec.com
It's an addicting site, almost as good as retrosheet.org for the baseball fan. Start at the top, you can find the current champs, ratings, and past fights listed by fighter name, be prepared to spend a couple hours there.
The four major organizations are the WBO, WBC, WBA, and the IBF. Most times, it's not in their best interests to work together, even with 17 seperate weight classes, there aren't many guys holding more than one organizations belt. Your two main promoter's are Bob Arum and Don King. They make the sport tick, I am sad to say. Matches are often made favoring their fighter's, not what is in the best interest of the sport or what the people want to see. Delahoya has his own promotion, they aren't big yet, but at least he has some integrity, and King and Arum ain't getting any younger, which is a good thing. When they are finally out of the picture, in the next ten years or so, you will see some major changes in the way boxing does business. Until then, it will still be a great sport, but a huge, confusing clusterfuck.