C & P'd from below
Courtesy of the links above:
BABIP - Batting Average on balls put into play. A pitcher's average on batted balls ending a plate appearance, excluding home runs. Based on the research of Voros McCracken and others, BABIP is mostly a function of a pitcher's defense and luck, rather than persistent skill. Thus, pitchers with abnormally high or low BABIPs are good bets to see their performances regress to the mean. A typical BABIP is about .290.
EqA - Equivalent Average. A measure of total offensive value per out, with corrections for league offensive level, home park, and team pitching. EQA considers batting as well as baserunning, but not the value of a position player's defense. The EqA adjusted for all-time also has a correction for league difficulty. The scale is deliberately set to approximate that of batting average. League average EqA is always equal to .260. EqA is derived from Raw EqA, which is (H + TB + 1.5*(BB + HBP + SB) + SH + SF) divided by (AB + BB + HBP + SH + SF + CS + SB). REqA is then normalized to account for league difficulty and scale to create EqA.
OBP - On-base percentage. (H + BB + HBP) divided by (AB + BB + HBP + SF). For pitchers, OBP is on base percentage allowed.
SLG% - Slugging percentage (hitters) or slugging percentage allowed (pitchers). Total bases divided by at-bats.
OPS - On Base Percentage + Slugging Percentage
VORP - Value Over Replacement Player. The number of runs contributed beyond what a replacement-level player at the same position would contribute if given the same percentage of team plate appearances. VORP scores do not consider the quality of a player's defense.
BB/9 - Walks allowed per every 9 innings.
K/9 - Strikeouts per every 9 innings.
K/BB - Strikeout to Walk ratio
WHIP - Walks plus hits allowed per inning pitched.
wOBA - The statistic wOBA (weight on base average) is now available in the player pages, leaderboards, team pages, my team pages, and the projections. wOBA, created by Tom Tango, is a version of linear weights that has been weighted to fit an OBP scale. The weights have been properly adjusted by season and for the minor leagues by season and by league.
Linear Weights - This is kind of long winded, but go here for an explanation.
There's many more, we'll add as they come up.