Sure, so let's take 2013 as an example. Not sure how much you know about baseball, so I'll try to stay basic.
Major League Baseball (MLB) is the highest level of baseball in North America, with 29 US teams and 1 Canadian team. MLB is split into two leagues, the American League (AL) and National League (NL). There are a few small rule differences in the leagues, but essentially it's just an organizational system. At the end of the season, the winner of each league plays each other in the World Series to see who becomes the "world champion".
The AL is traditionally seen as a stronger hitting league, due to the fact that pitchers don't hit (they do hit in the NL). Pitchers are not strong hitters (that's not really their job!), so therefore their batting averages are very low (bringing down the league average as a result).
Batting average is calculated by taking the total number of hits divided by total number of at bats. In 2013, you will see that the AL had an overall batting average of .256 and the NL had a .251. This means that in the AL, batters had a hit in 25.6% of at-bats. This is different, of course, than On-base Percentage (OBP), which is generally higher than BA because it accounts for walks and errors as well.
Let's take a good hitter as an example, Robinson Cano. http://www.baseball-reference.com/playe ... html?redir
Robinson had a total of 681 plate appearances (total number of opportunities to hit) in 2013, of which 605 counted as "at-bats". Batting average is calculated using "at-bats". Robinson had 190 hits, so therefore 190/605=.314, or a hit 31.4% of his at-bats. His OBP, which is calculated from total plate appearances and also considers errors and walks, was (190+65+6)/681=.383.
Hope this isn't too confusing or basic?