What is the difference between the various types of olive oils? In particular, what does it mean for an oil to be Extra Virgin versus just being Virgin? How do these compare to Extra Light?

I ask because I can tolerate the taste of Extra Light Olive Oil much better than I can the Extra Virgin kind. If it's just a matter of the Extra Virgin being better for me I can handle that. But if the Extra Light is either no good or, worse, harmful then I need to know.

TIA