Wow... That's a VERY vague question, and we could probably spend the remainder of the season answering it...
The Basics:
Fantasy Sports allow you to act as an owner of a fantasy sports franchise... At the beginning of the season (in most leagues) all the owners in the "league" draft players from the actual professional sports league by taking turns picking players from the ranks of the real sport to fill a fantasy roster... In football, after every team has 13-18 players, the teams are set, and the season is ready to begin... From your 13-18 person roster, each week you choose players to fill fantasy roster spots... Generally those spots are:
1 QB
2 RB
3 WR
1 TE
1 K
1 Team DEF
These spots vary from league to league, but that's a basic set-up...
In most leagues each team plays one other team every week, and scores for each team are based on the statistics that each player on your starting roster put up for that week... Typical scoring is something like this:
QB: 50 yards = 1 point, 1 TD = 6 points
RB / WR / TE: 10 yards = 1 point, 1 TD = 6points
K: 1 extra point = 1 point, 1 field goal = 3 points
DEF: 1 sack = 1 point, 1 turnover = 2 points, various points scored based on oppositions total points scored
Again, this varies greatly, but that's a basic scoring system...
After the full week of games in the real NFL, the total scores of each fantasy player's roster is tallied, and the person who scores the most points (between the two fantasy players that are matched up that week) gets a win... The looser, obviously gets a loss...
Towards the end of the season, the worst teams are eliminated, and the best teams face eachother in a playoff tournament, that culminates in a Superbowl Game between the last two remaining teams...
During the course of the season, you can drop people off your fantasy roster and add other players, and you can trade players with other people in your league...
In every league there is a commissioner that oversees fair play and makes sure the league runs smoothly, and settles the inevitable disputes...
That's the basics... I could go on for 30 more pages about the ins and outs of Fantasy Football alone, and there are similar fantasy rules for just about every other professional sport, as well...
Hope that helps...
This link might give you more insight:
http://football.about.com/od/fantasyfoo ... asy101.htm