Win
From Academic Kids

In baseball, a pitcher is credited with a win (or "W") when, in a game won by his team, he is the team's pitcher at the time that his team takes a lead that it does not relinquish for the remainder of the game.
An exception to this is that a pitcher who starts a game cannot be credited with a win if he does not pitch at least five innings. If his team takes a lead it does not relinquish while that pitcher is in the game, the win is awarded to the relieving pitcher who, in the judgment of the scorer, pitched the most effectively. The winning pitcher cannot, however, be a pitcher who is credited with a save in the same game.
A loss (denoted by "L") is charged to the pitcher who, in a game lost by his team, is charged with allowing the run that gives the opposing team a lead they do not relinquish for the remainder of the game. The pitcher who gives up a hit to score the "goahead run" does not necessarily receive the loss, it goes to the pitcher who allowed the runscoring player to reach base.
For example, in a tie ball game, if pitcher A walks a batter and then is relieved by pitcher B who then gives up a home run, the first run allowed is recorded to pitcher A and the second to pitcher B. Thus, pitcher A gave up the run that gave the opposing team the lead, and pitcher A will be credited with the loss if the other team does not relinquish the lead, even though pitcher B was on the mound when the goahead hit occurred.
Every game (excluding the rare tie game) has both a winning and a losing pitcher. A pitcher's total wins and losses are commonly noted together; a pitching record of 1510 indicates 15 wins and 10 losses.
A pitcher's winning percentage, standardly expressed to three digits, is calculated by dividing their wins by their total decisions (wins plus losses), as in the formula:
W / (W + L)
Records
Career wins
 Cy Young  511
 Walter Johnson  417
 Grover Cleveland Alexander  373
 Christy Mathewson  373
 Pud Galvin  364
 Warren Spahn  363
 Kid Nichols  361
 Tim Keefe  342
 Roger Clemens  333
 Steve Carlton  329
 John Clarkson  328
 Eddie Plank  326
 Nolan Ryan  324
 Don Sutton  324
 Phil Niekro  318
 Gaylord Perry  314
 Greg Maddux  311
 Tom Seaver  311
 Charles Radbourn  309
 Mickey Welch  307
 Lefty Grove  300
 Early Wynn  300
(See: Top 100 winning pitchers of all time)
Career losses
 Cy Young  315
 Pud Galvin  302
 Nolan Ryan  292
 Walter Johnson  279
 Phil Niekro  274
 Gaylord Perry  265
 Don Sutton  256
 Jack Powell  253
 Eppa Rixey  251
 Bert Blyleven  250