The good people at Pro Football Focus spend enormous amounts of time breaking down every player’s performance on every individual play throughout the season. In the end, players can then be given a final rating somewhere between zero (poor) and 100 (elite). If you want to learn more about their methodology, you can read PFF’s Player Grade overview.
Here’s a portion of what you will read if you click on that link:
On every play, a PFF analyst will grade each player on a scale of -2 to +2 according to what he did on the play.