Some of The Statistics I Use Explained

ERA+:

ERA+ is a comparison of a pitcher's ERA to league average. In 2011 league average in The American League was 4.08 and average in the National League was 3.81.  In the statistic ERA+ 100 is used as average and the percentage that the pitcher's ERA is better is added to 100. So if Roy Halladay had an ERA of 2.35 and League average was 3.81 his ERA+ would be 163. He is 63% better than league average.

OPS+:

OPS+ is also a comparison stat, but used to evaluate batters. In 2011 league average in the American League and National League was .730 and .710 respectively. 100 is also used as average and the difference between OPS+ and 100 is the percentage of players they are better than. Remember that OPS is on base plus slugging percentage. Miguel Cabrera had an OPS of 1.033. So his OPS+ is 181. He is 81% better than the average hitter.

Ground Ball Percentage:

For a pitcher getting batters to hit ground balls is great. They are more likely to hit into double plays and you don't risk the extra base hit. Basically you are more likely to get the out on a ground ball than a fly ball. In 2011 CC Sabathia had a GB% of 46.6%; which is great. Almost half of the hitters he faced are hitting ground balls. Sabathia also plays a lot of his games in Yankee Stadium where fly balls
 could hurt you a lot.

Fly Ball Percentage:

Fly Ball percentage is the ratio of fly balls hit to at bats. You want to hit fly balls as a hitter, they are more likely to score a run and get on base. Prince Fielder had a fly ball percentage of 37.1% in 2011. He was hitting a fly ball in more than one out of 3 at bats. If there is a runner on third and one out an opposing team may consider walking Fielder, because he could hit a fly ball to score the run.

Pitcher Performence Value:

A pitcher's performance can change by the team he faces and many other factors. A simple evaluation of a game is ERA before the game divided by the average OPS of the players he got out. If a pitcher gets out great batters but has an average ERA he is better than a pitcher who has an above average ERA and gets out mediocre batters. PPV = ERA/ AOPS

Run Production Probability:

What are the chances of a batter scoring a run in the inning he comes up to the plate? (R + RBI's - HR) / PA.
If Curtis Granderson were to come up to the plate in the third innings he would have about a 32% of scoring a run in that inning. Since a player gets about 4 at bats in a game the chances that Curtis Granderson will produce a run in that game are very likely, making him valuable to an offense.