Have trouble understanding the theory behind park effects? It's a quite simple idea; in essence, there aren't standardized stadium sizes in baseball, so what may be a double in one park might be a fly out in another. So in order to compare statistics between players, we should adjust for these differences or else we'll end up weighing 30 home runs in Coors the same as 30 home runs in PETCO park.
I like to think that's pretty easy to understand but in case you want a bit of visual proof, check this out:
This is obviously a bit of an extreme example since Fenway Park is such an odd place, but I can count four doubles and four fly-outs that would have been home runs last year if Longoria's home park had been Fenway. That'd have put his season total at 41 home runs, which is a bit different than the 33 he actually hit.
Just to clarify, this isn't actually how park effects are calculated. Someone doesn't overlay hits on all sorts of different parks and attempt to determine adjusted lines based on a visual perusal. It involves complicated math and regressions and stuff, so check out Graham's article on park effects if you want more information.
Anyway, I could do this all day. I'm using this excellent new tool developed by Sean over at Katron.org, and it's one of the most fun things that I've run across online in a long time. Fool around on your own if you want and explore some of the different overlays you can do. It's a cool way to get to understand the idea of park effects and it also gives you nice insight on where your favorite players tend to hit. If you're looking for something to get you started, try looking at how Adrian Gonzalez would do in Fenway. Terrifying! Absolutely terrifying.