You draft a team of players.. much like franchise mode in most football games if you start from the draft, or imagine every football player in a rookie draft.
Each week, you play certain people of your team, based on how they're doing, if they're injured, not playing that week, etc..
You also can trade your players for other players, release players who are injured for the rest of the season, emerging stars, backups, etc.
Each week (sometimes every day if you're that into it), you set and change your lineup to reflect the upcoming round of football being played. The players you have starting earn points on how well they play, such as your starting runningbacks (usually you have 2 starting), if they run for 100 yards, have 50 yards recieving, score a few TDs.. you get points from that. You can also lose points if they fumble, etc..
Don't know what else I'm missing.. there has got to be something..
Pretty fun if you're into football if you ask me.
