Explanation of Baseball Statistics
Batting Average (AVG)
Divide the number of base hits by the total number of at bats. If Tony Gwynn has 600 at bats and has 206 hits, His batting average would be .343 (206/600).

Earned Run Average (ERA)Multiply the total number of earned runs by nine, and divide the results by the total innings pitched.
Randy Johnson has allowed 67 runs in 220 innings. What's his ERA?. Multiply 67 by 9: 67x9=603. Divide 603 by 220 (his innings pitched): 603/220=2.74. That's his ERA. 
WL PercentageDivide the number of games won by the total number of decisions. Pedro Martinez has a 163 record. Divide his win total(16) by his total number of decisions (19): 16/19 = 0.842

Slugging Percentage (SLG)Divide the total number of bases of all base hits by the total number of times at bat. Sammy Sosa has 282 total bases and 440 atbats. Divide 282 by 440 to get his slugging percentage: 282/440 = .641

OnBase Percentage (OBP)Divide the total number of hits plus Bases on Balls plus hits by Pitch BY at Bats plus Bases on Balls plus hit by Pitch plus Sacrifice Flies. In Derek Jeter's 434 atbats, he has 152 hits, 59 walks, and has been hit by 9 pitches. He's hit 6 sacrifice flies.
So here's the formula to determine his onbase average: (152+59+9)/(434+59+9+6) = 220/508 = .433 
Fielding AverageDivide the total number of putouts and assists by the total number of putouts, assists and errors. Edgardo Alfonzo has 218 putouts and 290 assists, while committing only 2 errors.
So his fielding percentage is: (218+290)/(218+290+2) = 508/510 = .996 