Strikeouts (K): The number of strikeouts recorded by a pitcher. A strikeout occurs when a pitcher throws any combination of three swinging or looking strikes to a hitter.
Hits Allowed (HA): The number of hits a pitcher gives up. A hit occurs when a batter strikes the baseball into fair territory and reaches base without doing so via an error or a fielder's choice. If a player is thrown out attempting to take an extra base (e.g., turning a single into a double), that still counts as a hit. Walks do not count towards hits allowed.
Earned Runs (ER): The number of earned runs scored against a pitcher. An earned run is any run that scores against a pitcher without the benefit of an error or a passed ball. If a pitcher exits a game with runners on base, any earned runs scored by those runners will count against him.
Outs (OUTS): The total number of team outs recorded while a pitcher is in the game. An out is recorded when a player at bat or a baserunner is retired by the team in the field.
Walks (BB): The number of batters walked by a pitcher. A walk (or base on balls) occurs when a pitcher throws four pitches out of the strike zone, none of which are swung at by the hitter. The walks stat includes intentional walks.
Singles (1B): The number of singles hit by a batter. A single occurs when a batter hits the ball and reaches first base without the help of an intervening error. Doubles, triples, and home runs do not count towards the singles stat.
Total Bases (BASES): The number of bases gained by a batter through his hits. A batter records one total base for a single, two total bases for a double, three total bases for a triple and four total bases for a home run. Walks, errors, and hit by pitch occurrences do not count towards total bases.
Hits (HITS): The number of hits by a batter. A hit occurs when a batter strikes the baseball into fair territory and reaches base without doing so via an error or a fielder's choice. There are four types of hits in baseball: singles, doubles, triples and home runs. If a player is thrown out attempting to take an extra base (e.g., turning a single into a double), that still counts as a hit. Walks do not count towards hits.
Runs (RUNS): The number of runs scored by a player. A player is awarded a run if he crosses the plate to score his team a run. When tallying runs scored, the way in which a player reached base is not considered. If a player reaches base by an error or a fielder's choice, as long as he comes around to score, he is still credited with a run.
Runs Batted In (RBI): The number of runs batted in by a player. A batter is credited with an RBI in most cases where the result of his plate appearance is a run being scored. There are a few exceptions, however. A player does not receive an RBI when the run scores as a result of an error or ground into double play. The most common examples of RBIs are run-scoring hits. However, players also receive an RBI for a bases-loaded walk or hit by pitch. Players can earn RBIs when they make outs, as well, provided the out results in a run or runs (except, as noted above, in the case of double plays).