[ryanivr]

To explain: Both ERA and opponent's AVG are heavily influenced by the defense behind them. They are largely influenced by luck.

There's a LOT of noise when a hitter hits the ball in play. To evaluate a pitcher, you want to eliminate as much of that noise as possible. That's what FIP does. It removes the defense from the equation and relies on the things a pitcher can control; Strikeouts, Home Runs Allowed, and Walks.

FIP assumes that balls hit in play were league average, and tells you what the pitcher's ERA should have been if that were true.

You can use this along with ERA to determine a few things. If a pitchers ERA is 2.50 and his FIP is 4.00, you're looking at a pitcher that got very lucky OR had a tremendous defense behind him. If you have a guy whose ERA is 5.20 but his FIP is 3.75, you're looking at a pitcher who probably gives up a lot of singles and has a pretty lousy defense behind him. You can then use that to determine a general expectation for the future.