The Player Efficiency Rating (PER) is a rating of a player's per-minute productivity.
To generate it, I created formulas -- which I've outlined in tortuous detail in "Pro Basketball Forecast" -- that return a value for each of a player's accomplishments. That includes positive accomplishments, such as field goals, free throws, 3-pointers, assists, rebounds, blocks and steals, and negative ones, such as missed shots, turnovers and personal fouls.
Two important things to remember about PER is that it's per-minute and pace-adjusted. It's a per-minute measure because that allows us to compare, say, T.J. Ford to Jose Calderon, even though there is a wide disparity in the minutes they played. I also adjust each player's rating for his team's pace, so that players on a slow-paced team like Detroit aren't penalized just because their team's games have fewer possessions than those of a fast-paced team such as Golden State.