A park factor is a measure of how a team's home field changes its statistics. It results from a combination of many factors -- the distance and height of the outfield fences, angles, foul territory, visibility, field surface and weather, for example.
It is not the case that the Yankees have a high park factor for home runs (PF) because they hit a lot of home runs. To get a high PF, you need to hit and allow more home runs in your home games than you do in your road games. An average effect on HR is written as '100'; a better-than-average park will score something like 120, which means they get a boost 20 percent above average, while a poor hitters' park would score 90, or 10 percent below average. Players are also graded on who has the best and worst fit to their stadium -- not on how well they hit home/road, but how well their profile matches or doesn't match what the park gives.
A look at the parks in this year's postseason:
To see a park-by-park breakdown -- including why a Twins/Yankees series could lead to another frustrating postseason for A-Rod, and one reason why the Red Sox always seem to do so well when visiting the Angels -- you must be an ESPN Insider.