All batter lines will be listed as AVG/OBP/SLG
AVG = Batting Average on balls in play
OBP = On Base Percentage, measures a players ability to get on base via hits, walks or hit-by-pitches
SLG = Slugging Average, measures a players ability to hit for power (in AVG, a home run and a single count the same; in SLG a player is credited for each base reached, 1-single, 2-double, etc.)
Another metric that I will often use is OPS+, which looks at a player’s ability to create runs, while adjusting for park and league factors, and comparing that player to the rest of the league.
For example: an OPS+ of 100 means the player was league-average at creating runs, 95 means the player was 5% worse at creating runs, and 110 means the player was 10% better at creating runs.
By the same token, I will also use ERA+ for pitchers, which indexes the pitchers ERA against the league average, taking into account park and league factors.
For a more detailed look at how statistics can enhance our understanding and evaluation of the game of baseball, please check out the Baseball Prospectus Basics Series.