In the world of advanced basketball analytics, offensive and defensive ratings are used as a way to track and compare how an individual player affects his or her team's performance. At a basic level, the offensive rating of a player represents how many points a team scores when that player is on the court and the defensive rating for how many points they allow. The two combined (offense-defense) are referred to as the net rating often times written as NETRTG.
Net rating is the offensive rating minus the defensive rating, but simply put it can be defined as how much better or worse the team is when a specific player is on the court. These ratings are usually on a per X possessions basis. Using possessions rather than minutes eliminates the effects of a team that plays very fast or very slow.