Wins Above Replacement Player
Wins Above Replacement Player, or WARP, is a statistic published in Baseball Prospectus that attempts to measure the "total value" of a player over a given season.
WARP, intuitively, attempts to express the total number of wins that a given player adds to his team over the course of a season by comparing the player's performance with that of a fictitious "replacement player". A "replacement player" is assumed to be an average Triple-A callup who might appear in the majors only as replacement for an injured player, and whose hitting, fielding, and (if applicable) pitching skills are far below league average. According to Baseball Prospectus, a team consisting entirely of replacement-level players would likely be historically bad, winning only 20-25 games over a full 162-game season.
To compute WARP, Baseball Prospectus uses three other proprietary statistics: Batting Runs Above Replacement (BRAR), Fielding Runs Above Replacement (FRAR), and Pitching Runs Above Replacement (PRAR). The three numbers are added and divided by the number of runs per win that season (another proprietary number. In recent years, this number is around 10).
Most regular position players will accumulate 3-5 WARP over a season. A legitimate All-Star-caliber player may have over 7 WARP. Over 10 WARP is a strong MVP candidate, while over 15 WARP is a "one-for-the-ages" season. On the flip side, a player with -1 WARP or less is probably in danger of disappearing from baseball.
Teams can also be ranked by their cumulative WARP. A team with a total of 30 WARP or less among all players would be a disastrous, certain last-place finisher. 50-60 WARP can be expected from a .500 team. 65-70 WARP is a playoff-caliber team, while much more than that would be a strong World Series candidate.
WARP2 and WARP3
Baseball Prospectus also provides variations of WARP that can be used to compare players across leagues and seasons. WARP1 is the statistic described above, and is best used when comparing players within a certain league and year. WARP2 adjusts WARP1 by the difficulty of playing in a particular league, and WARP3 adjusts WARP2 by season length. Therefore, WARP2 is the statistic intended for use when comparing player careers of different eras, while WARP3 is best used for comparing single seasons across different years.
Many long-time, successful players can accumulate over 50 WARP2 for a career; strong Hall of Fame candidates typically have well over 100.