For those that haven't played,
There are 10 categories for which you get points. R, HR, RBI, Avg, SB for batters and W, S, K, Whip, and ERA for pitchers.
Each team drafts a batter from the MLB for each position. So, you'll have 1 1B, 1 2B, 1 SS, 3 OF etc. And usually a Util in which you can put any hitter you like in. So you try to assemble the best team you can to maximize the amount of points you get. You get points based on your stats compared to other league members, so for example if I have 25 HR's and trim has 24 Hr's I would be beating him in that category and would get more points. If I'm winning the category in the entire league, I would get the maximum amount of points.
Same goes for pitching. If I have an ERA of 3.4 and Trim has an ERA of 3.7 I would be getting more pts than Trim in the ERA category (since ERA is better the lower it is).
Sounds difficult now that I've written it all out, but it's terribly easy. I'm sure you could check out fantasysports.yahoo.com and check out the baseball rules somehow.
Each day you would choose, based on your roster, who to play based on their matchups etc. Each day you can also cut/add players to your liking. At the beginning of the year we have a draft in which every manager picks the players from the MLB he wants on his team.