The plus-minus, a.k.a. +/-, simply keeps track of the net changes in score when a given player is either on or off the court.
NBA.com has been publishing plus-minus values in official box scores since 2007-2008 season.
Since the invention of basic plus minus, the plus-minus technique has been developed in order to get more accurate results.
There have been some stat heads who have developed different types of plus minus metrics. Here’s a collection: