Disclaimer: This piece is entirely my own work, no cutting or pasting involved. It is also a work in progress, as there are a lot of statistics in baseball.
ESPN.com and
cbs.sportsline.com were used as references.
A fairly comprehensive list of abbreviations is available on the Baseball Statistics Abbreviations node.
HITTING
A Plate Appearance (PA) is any occasion on which a player comes to the plate to bat. The only exception is when a player comes to the plate and is then pinch-hit for before he gets a chance to bat.
An At-Bat (AB) is a plate appearance during which the player collects a base hit or records an out. If the player draws a walk, is hit by pitch, or records a sacrifice, then the plate appearance is not an at-bat.
Batting Average (AVG or BA) is the percentage of a player's at-bats which result in base hits. The number is generated by dividing the player's total number of hits by the player's total number of at-bats. A batting average in the mid .200's is average. Anything over .300 is considered very good. A season-long average over .400 has not been achieved in decades, although Todd Helton flirted with the feat before hitting a slump towards the end of the season.
Hits (H) are achieved by putting the ball in play and reaching base without the benefit of a Fielder's Choice or an error. A hit wherein the batter stops at first base is also called a single.
Doubles (2B) occur when the batter gets a hit and reaches second base without the benefit of an error or being able to advance a base due to a fielder's throw to another base.
Triples (3B) occur when the batter gets a hit and reaches third base without the benefit of an error or being able to advance a base due to a fielder's throw to another base.
Home Runs (HR) occur when a batter gets a hit and scores on the same play without the benefit of an error or being able to advance a base due to a fielder's throw to another base. Most home runs happen as a result of a player hitting the ball out of the field of play in fair territory, but one can also achieve an "inside the park" home run.
Base on Balls/Walks (BB) occur when the pitcher throws four balls (pitches not in the strike zone and not swung at by the batter). The batter is awarded first base, and if there is a runner at first, he takes second. If there are runners at first and second, then each runner moves to the next base. If the bases are loaded, the runner standing at third base scores, and the batter is awarded an RBI.
Intentional Walks (IBB) occur when the pitcher and catcher elect to purposely put the batter on first base rather than pitch to him. This is usually done to avoid the best hitters on the team in an important situation, or to reach the weakest hitters on the team in such a situation.
Hit By Pitch (HBP) is granted if a player is struck by the ball thrown by the pitcher, despite his efforts to get out of the way. If the umpire decides that the player did not attempt to get out of the way of the ball, he has the power to not grant the HBP. The results of an HBP are the same as a walk.
Grounded Into Double-Play (GIDP) is added to a player's stats any time he puts a ball in play which results in two outs. If a runner scores on a double play, the batter is not awarded an RBI.
On-Base Percentage (OBP) is the percentage of a player's plate appearances which result in the player reaching base. It is calculated by adding together the players total hits, total walks, and number of times hit by pitch, then dividing that number by the player's total number of plate appearances.
Total Bases (TB) is, naturally, the number of bases a player reaches through his own hits. Stolen bases, bases taken on errors, and bases taken during a play resulting from another player's plate appearance are not counted in this statistic. It can be calculated by adding the player's total number of singles hit, plus twice his number of doubles hit, plus three times his number of triples hit, plus four times his number of home runs hit.
Runs Scored (R) represents the number of times a player crosses from third base to home, resulting in a run scored for his team.
Runs Batted In (RBI) is the number of times a teammate scores as a result of a ball hit in play by a particular player. Home runs result in at least one RBI (a Grand Slam results in four RBI) because the player bats himself in. If a teammate scores on a play in which the batter grounds into a double play, the batter is not awarded an RBI.
Slugging Percentage (SLG) is the most important statistic in power hitting. It is, in effect, the average number of bases a player reaches each time he comes to bat. For example, if a player strikes out twice, and then hits a double in his third at-bat, he has collected two bases in three at-bats, so his slugging percentage is .666. The percentage is calculated by the player's total bases divided by his number of at-bats.
On-Base Plus Slugging (OPS) has become a popular statistic for rating a hitter's overall performance. It is simply the player's On-Base Percentage added to his Slugging Percentage.
PITCHING
Earned Runs (ER) are runs scored by the opposing team which are the responsibility of the pitcher. Runs that score on fielding errors or passed balls (balls that should be caught by the catcher but get past him) are not counted as earned. If a fielding error occurs on what should have been the third (and final) out of an inning, all runs from that point on scored in the inning are unearned, regardless of how they are scored.
Innings Pitched (IP) is, obviously, the number of innings pitched by a pitcher. Every out in the game is one third of an inning.
Earned Run Average (ERA) is the average number of earned runs given up by a pitcher in a nine inning game. The number is determined by multiplying the pitcher's total number of earned runs by nine, then dividing that number by his innings pitched. Anything under a 3.00 is excellent. Anything under a 4.00 is good. ERA's ranging from 4.00-5.00 are fairly average. Note: Pitchers in the American League tend to have higher ERA's than those in the National League. This is a result of the Designated Hitter, meaning that pitchers, generally the weakest hitters, do not have to hit for themselves in the American League, so the pitchers must face a line-up of nine legitimate hitters.
Walks and Hits per Innings Pitched (WHIP) is just what the name says. It is calculated by adding together a pitcher's walks and hits allowed and dividing by his total innings pitched.
Wins (W) are earned in the following manner: If a pitcher starts the game and pitches at least 5 innings, then leaves the game while his team is ahead, and his team does not relinquish the lead for the rest of the game, he is awarded with a win. He does not earn a win if he leaves the game with a tied or losing score, even if his team wins.
A relief pitcher who leaves the game at any point after the 5th inning is rewarded a win if he entered the game when his team was tied or losing, and leaves with them winning, and they win the game without relinquishing the lead.
Losses (L) are given in the following manner: If a pitcher leaves the game with his team losing and they do not tie or take the lead in the game at any point after his leaving, he is given a loss. If the team at any point after his leaving does tie or take the lead in the game, the pitcher is off the hook for the loss, regardless of wheter his team wins or loses.
Saves (S) are awarded when a relief pitcher accomplishes one of three things: 1) He inherits a lead of 3 runs or less, and finishes out the game by pitching at least one full inning without giving up the lead or the lead increasing to 4 or more. 2) He finishes the game by pitching 3 or more innings in relief with a lead of any size and not relinquishing it at any point. 3) He inherits a situation where the potential tying run is on base, at bat, or waiting to bat next.
A player cannot record a win and a save in the same game. A player cannot "pitch himself into a save." In other words, if a player inherits a 5 run lead, but gives up 3 runs to dwindle the lead to 2, he is not in a save situation (although any pitcher who relieved him would be).
Holds (H) are awarded to a relief pitcher who enters the game in a save opportunity, records at least one out, and leaves the game without surrendering the lead.
Quality Starts (QS) is a statistic often scoffed at. Basically a creation of agents to use as a bargaining tool, a pitcher is awarded a quality start when he starts a game and pitches at least 6 innings without giving up more than 3 earned runs.
FIELDING
Put-Outs (PO) are outs recorded directly by a fielder, either by tagging the runner out, or by stepping on a base resulting in a force out.
Assists (A) are rewarded when a player throws the ball to a team mate who records an out.
Errors (E) are recorded when a player should make a put-out or assist, but fails. This can be a very subjective statistic, and errors (unless glaring) are generally judgment calls by the official scorer.
Total Chances (TC) is the number of times a ball is put in play in such a way that an out or assist should be recorded by a particular fielder.
Fielding Percentage (FPCT) is the percentage of total chances converted. If X is the sum of assists and put-outs, then fielding percentage is calculated by dividing X by (X+Errors).
Double Plays (DP) are recorded when two outs are made on the same play. DP's are rewarded to every player who makes an assist or a put-out on the play.
Triple Plays (TP) are very rare. Generally only one or two happen each season. They are similar to double plays, except that three outs are recorded on the play. Triple plays are so rare that you rarely see this stat listed.