Olive oil is a fat obtained by the cold pressing of olives (the fruit of the olive tree). Rather like wine, olive oil comes from a vast array of origins and cultivation methods. In the bottle, this offers many different bouquets, flavours and colours. So there is not just olive oil, but lots of olive oils, each with its own special character.
Note: the comprison with wine only goes so far; unlike wine, olive oil does not improve with age.
If you intend to buy: Choose a high-quality oil of known origin. Beware of oils at bargain prices; they are rarely a good deal.
I invite you to take a look at my good addresses page, where you can find the contact details of a producer in Crete whose oil I particularly recommend.