There are a number of differences, but most importantly, OBP includes walks and hit-by-pitch in the numerator AND demoninator, while simple averages excludes them from the calculation.
Imagine a batter who comes to the plate 10 times, and gets 3 hits and a walk.
His batting average is .333 (3 divided by 9), as the walk is excluded from batting average, as if it never happened. Thus while he had 10 plate appearances, he only had 9 "at-bats" for purposes of computing average.
The hitters OBP is .400, 4 divided by 10. 3 hits + a walk = 4, divided by plate appearances.
Wikipedia has a more expansive discussion here:
Cheers mate, thats a great help! I'm really glad I've found a place to discuss baseball with people who know what they are talking about and glad to help. As you might imagine, there are not too many people with whom I can talk baseball with across the pond, we are sadly in the enlightened minority!