wRC+ takes the statistic Runs Created and adjusts that number to account for important external factors -- like ballpark or era. It's adjusted, so a wRC+ of 100 is league average and 150 would be 50 percent above league average.
For example, a player who plays his home games at hitter-friendly Coors Field will have a lower wRC+ than a player who posts identical stats at pitcher-friendly O.co Coliseum. The production of the player at Coors Field is deemed less impressive because of his ballpark's hitter-friendly nature.
(((wRAA per PA + league runs per PA) + (league runs per PA - ballpark factor x league runs per PA) / league wRC per plate appearance, not including pitchers)) x 100.
Why it's useful
wRC+ quantifies the most important part of a batter's job -- creating runs -- and normalizes it, so we can compare players who play in different ballparks and even different eras.
wRC+ is about as good as it gets when it comes to assessing hitters in a vacuum, but fantasy leagues don't adjust for ballpark or league -- making it a slightly less useful tool in the fantasy realm.