What follows is a basic, semi-literate summary of some of the more advanced stats I refer to. I'm assuming a knowledge of batting average, earned run average, and all the other default stats. Enjoy.
On-Base Percentage: Quite simply, how often a player gets on base; hits, walks, and hit-by-pitches. OBP vs. batting average is the most basic front on the fight between stat dorks and traditional fans. OBP's importance is almost a philosophical issue: a baseball game is not limited by time or points. It's limited by outs. The longer you can go without making outs, the more runs you have the opportunity to score, and OBP measures how often a player makes outs. No, a walk is not as good as a hit, but a bunt single isn't as good as a triple, and batting average makes no distinction there. .400 is an excellent OBP, and the career leader is Ted Williams at .482.
Slugging Percentage: Total Bases divided by At-Bats. SLG gives you an idea of a player's power. If you hit a homer every time up, your SLG is 4.000. .500 is the gold standard, and Babe Ruth's career .690 is the best ever. Typically I'll express a players batting line in batting average/on-base percentage/slugging percentage format; Brad Wilkerson was at 255/374/498 last year.
OPS is On-base Plus Slugging. It's a handy, quick-and-dirty summation of a player's offense. Helpfully, it sort of works like a grade: 900 is an A, 800 is a B, and so on. The problem with OPS is that a point of OBP is actually more valuable than a point of SLG. In other words, if you have two players both with an 800 OPS, the guy with an OBP/SLG of 400/400 is more valuable than the one with 350/450. Ruth is the career leader at 1164.
OPS+ improves on OPS, the other problem of which is that it treats all OBPs and SLGs the same regardless of when or where they occur. An 800 OPS is more impressive in 1968 than in 1998, and better in Dodger Stadium than in Coors Field. OPS+ compares a player's OPS to the league and rates it with 100 being average. Ruth's career 207 leads the pack, and Wilkerson put up a 128 in 2004.
VORP: Value Over Replacement Player is a Baseball Prospectus concoction that measures how many more runs a player produces than a hypothetical "replacement player," let's call him "Dayn." Dayn really sucks, and it's tough for a guy who gets any kind of playing time not to outproduce him. VORP is adjusted for park, league, and position (.350 OPB gets a shortstop more VORP than a first baseman). The really good thing about VORP is that it's a counting stat that's not bullcrap. Batting average, OBP, SLG, and OPS are rate stats - they don't take playing time into consideration. Hits, runs, RBI, et al are counting stats and are very dependent on playing time. None of that latter group, however, tells you much about an individual player's contribution, and VORP does. Barry Bonds led baseball with 142 VORP last year, and Wilkerson chimed in with 48.2.
Win Shares is something I don't use much because I don't trust it - people much brainier than I have detected problems in the methodology, particularly in the defensive aspect. Bill James came up with an all-encompassing stat that boils down a player's contribution at the plate, in the field, and on the mound into one handy number, one-third of a win. Here's a fun site with hella sortable Win Shares. Barry Bonds was first last year with 53, and Wilkerson chipped in with 22, tied with Curt Schilling and Jim Thome, among others.
Now, on to pitching.
ERA+ does to ERA what OPS+ does to OPS. ERA is affected by ballpark and various other factors. ERA+ adjusts for these and, like OPS+, 100 is average. Pedro Martinez is the all-time leader at 167. Lefty Grove leads inactive players 148, with our own Walter Johnson right behind with 146.
VORP works for pitchers too. It measures the runs a pitcher saves over a replacement pitcher, let's call him "Will." Will is a very bad pitcher; Jose Contreras was at Will-level last year with his 5.50 ERA. Here's why VORP is important: in 2004, Jake Peavy of San Diego had a good year. 2.27 ERA, 166.3 IP. That's certainly better than Livan Hernandez' 3.60 ERA, right? Wrong, sucker! Livan's 255 IP put him over the top, as he out-VORPed Peavy 58.3 to 57.5. So, yes, I like VORP because it makes Livan look good, like vertical stripes.
Win Shares also crosses over, but I don't like them for pitchers either. Johan Santana led all pitchers with 27, and Livan had 19.