Each week you face another team from the league. I believe there are 9 "scoring" criteria that gauge your players' performance throughout the week.
Whomever wins the most categories based off those points each week gets the "win" over your opponent. (eg, if you win 5 categories, he wins 4, you beat him and notch a "win" in the standings, he gets a loss).
We got two divisions right now, and I think 8 teams make playoffs, with top seeds, etc. That's of course based off how you rank in your division based off those wins/losses.
You can drop a player from your team and pick up another from "free agency"/waiver whenever; I think 4 moves like that are allowed per week. And you can change your lineup whenever you like, daily, rotating around guys already on your team; as long as a player on your roster hasn't already begun that day's game, you can bench him and replace him with someone from your bench or team whenever you'd like.