Start with this
for a primer.
UZR is widely regarded as the best system that's available, and if you really want to figure out how it all works you can read about that here
. The nice thing about UZR is that not only is it regarded as the best, it's also the system available at FanGraphs.
Typically the defensive numbers are subject to small sample size issues, so you usually need multiple years of data to draw any conclusions about an individual player. If I'm trying to get a sense of how good a player is, I'll usually look at three years from a few different systems (usually UZR and PMR
and some CHONE
thrown in for good measure. Then you should slap a range around it, so if those sources are pointing at a player being a +10 (run, so ~ 1 win) fielder, he's probably anywhere from a +5 to +15 fielder.
Then if you combine that with how good of a hitter a player is (based on wOBA, also available at FanGraphs), you can figure out just how many wins a given player is worth. It's awesome stuff.