Well, it's been a while since I've been in one (for hockey), but basically, you're a team's general manager, and you have a set amount of money, and you make your team, by selecting actual players.
Every time someone on your team does something in real life, you get points. The more a player does, the more a player is worth, the more you can sell him for, etc.
The more points your team accumulates, the bigger your score.