ERA+ takes a player's ERA and normalizes it across the entire league. It accounts for external factors like ballparks and opponents. It then adjusts, so a score of 100 is league average, and 150 is 50 percent better than the league average.
For example, Mariano Rivera's 2.21 career ERA was 105 percent better than the MLB average during the time he pitched (including adjustments for park and league). That gives him a 205 career ERA+ (the best all-time).
OPS+ takes a player's on-base plus slugging percentage and normalizes the number across the entire league. It accounts for external factors like ballparks. It then adjusts so a score of 100 is league average, and 150 is 50 percent better than the league average.
For example, Miguel Cabrera's .895 OPS in 2014 was 50 percent better than the MLB average after being adjusted for league and park factors. As a result, his OPS+ was 150.
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 Oakland Coliseum.
WAR measures a player's value in all facets of the game by deciphering how many more wins he's worth than a replacement-level player at his same position (Minor League replacements).
For example, Joey Votto spent 17 seasons with the Cincinnati Reds amassing 64.4 career WAR. This means that if Joey Votto would've never played for the Reds and been replaced with an average player, the Reds should have lost 64 more games in that 17 year span.