Eff48 is a statistic that has been compiled by the NBA since 2002. It measures the sum of all of the player's positive production stats (points + rebounds + assists + blocks + steals) subtracts them from the sum of all of the player's negative production stats (field goals missed + free throws missed + turnovers), then divides that by the minutes the player played, and then multiplies that number by 48.
This statistic provides a much, much better... albeit still not perfect... measure of a player's performance than you would traditionally get just by looking at the player's points and rebounds. Unlike those stats, Eff48 rewards all-around production, while it penalizes "volume scorers" -- guys who score a lot of points but cost their teams by missing a lot of shots.
I also like it because it is comprehensive yet very easy to understand, unlike some other more mathematically inclined "performance" statistics (see Hollinger, John.. "Per" stats).
Here's how to read the Eff48 numbers:
Eff48 Easy Translator
23+: Pretty good
< 20: Below Average
< 15: Crappy
< 10: NBDL Reservations being made