Extra Virgin Olive Oil is from the frist cold press (this nomenclature is dictated legally in Europe but in the US not really so the bottle shoud say "first cold press" Italy takes what they call their olive oil seriously
Other "olive oils" are from subsequent pressings
"light" olive oil is actually a blended oil with some olive oil content. It is not healthier or less calories or fat. I personally dont use it but some people like it for its taste and I believe it has a higher smoke point so can be preferred for frying.
Within "Extra Virgin" there is a great variation in qualilty and flavors ranging from the cloudy thick green stuff which is prescious as liquid gold and used mainly for flavoring/drizzling and not cooking to the stuff in the jumbo can. There is also EVOO from many countries like greece, turkey , spain etc all a bit different.... Honestly I usually buy what's on sale
here is a good link.
HowStuffWorks "Types of Olive Oil"
Sattie... I love World Market... we had one in Houston I miss it