Ratings

I am new to this site and I have to say that I am really having a great time. I really like being able to play the turn based games and of course the live chess. I am still having a hard time understanding the tactics trainer and exactly what I am supposed to be doing in each one. Maybe I'll get it figured out one day. My question is this, How are player ratings determined? I have noticed that in playing live chess and also in the tactics trainer that ratings go up and down. What are the guidelines that determine how much a rating goes up and down? I have always enjoyed playing chess and I don't know much about the history and all and up until recently, never really cared about ratings. I am just hoping that someone can shine a real clear light on this subject.

 Thanks.

Here's a great explanation:

http://www.chess.com/article/view/chess-ratings---how-they-work