What is the difference between batting average and on-base percentage?

Batting average counts hits against at-bats, and an at-bat is what is left after walks, hit-by-pitches and sacrifices are taken out — so a walk moves neither half of the fraction. On-base percentage asks the broader question: how often did the batter avoid making an out, by any means. A patient hitter who walks 100 times can sit 60 points of OBP above a free swinger with the same average, and it is the second number that predicts runs.

Batting Average (AVG) in fullOn-Base % (OBP) in full