From BR Bullpen
Park factor is a formulaic calculation that seeks to show how much players on a particular team were helped by playing in their home park for half the games each season. For example, it is well known that Coors Field is a much easier place to hit than most ballparks, and "park factor" seeks to quantify that. The idea is to "let the air out of" statistics accomplished in easy ballparks, and give more credit to players who had to play in ballparks where no one could hit well due to factors intrinsic to the ballpark in question.
The main section of Baseball-reference.com lists the park factor for each team for each year. As an example, if you look at the 2005 New York Mets page, you will find the park factor listed near the top of the page, underneath the lines for "ballpark" and "attendance" statistics.
A park factor of 100 is exactly neutral, while a park factor over 100 is favorable to batters. A park factor under 100 is unfavorable to hitters. The farther a park factor deviates from 100, the more extreme the park is. Park factors are calculated for both batters and for pitchers (and are usually pretty similar).