Net plus-minus, a.k.a. unadjusted plus-minus, is invented by Roland Beech and measure the plus-minus statistics for a given player when the player is in the game relative to the plus-minus statistics when the player is NOT in the game. Net plus-minus combines offensive and defensive plus-minus.

For example, if a team scores 115 points (per 100 possessions) while a given player is on the court and 98 points (per 100 possessions) while he is off the court, his net offensive plus-minus is +17 (115-98=17).
If the team allows 110 points (per 100 possessions) while given player is on the court and 105 points (per 100 possessions) while he is off the court, his net defensive plus-minus is +5 (110-105=+5).
So, net plus-minus for the given player is +17-(+5)=+12