I've done football,basketball,baseball and hockey on yahoo at one time or another.
They put you in a division, and give you your draft position,then you draft the players you want(if no one else gets them first).
You get points by how well your players do in real life.
And you can trade players with other people.
Football is the easiest one to play,since you don't have to worry too much about your line-up. (You don't have games everyday or every other day where you have to constantly tweak your line-up).