The difference between the olive oils is in their acidity level, which affects mostly taste, not nutritional content. Lower acidity oils, such as extra virgin, tend to have more anti-oxidants, but that is not reflected in their classification. Anti-oxidants in olive oil may help prevent heart disease and cancer so sticking with extra virgin seems to make sense. Pomace olive oil is processed with hexane and other solvents just as most seed oils like canola, corn, safflower, soy, etc. This removes many of the minor constituents which may be the healthiest part of this natural product. http://www.oliveoilsource.com/cooking_olive_oil.htm

Virgin Olive oil is from riper olives and has a higher acid content (1%) than EVOO (extra virgin olive oil).

Extra-virgin olive oil comes from the first pressing of the olives, contains no more than 0.8% acidity, and is judged to have a superior taste. There can be no refined oil in extra-virgin olive oil. Virgin olive oil has an acidity less than 2%, and judged to have a good taste. There can be no refined oil in virgin olive oil. Olive oil is a blend of virgin oil and refined virgin oil, containing at most 1% acidity. It commonly lacks a strong flavor. Olive-pomace oil is a blend of refined pomace olive oil and possibly some virgin oil. It is fit for consumption, but it may not be called olive oil. Olive-pomace oil is rarely found in a grocery store; it is often used for certain kinds of cooking in restaurants. Lampante oil is olive oil not used for consumption; lampante comes from olive oil's ancient use as fuel in oil-burning lamps. Lampante oil is mostly used in the industrial market. http://en.wikipedia.org/wiki/Olive_oil

Extra-virgin olive oil (sometimes called EVOO) comes from the first pressing of the olives, contains no more than 0.8% acidity, and is judged to have a superior taste. There can be no refined oil in extra-virgin olive oil. Virgin olive oil has an acidity less than 2%, and judged to have a good taste. There can be no refined oil in virgin olive oil. Pure olive oil Oils labeled as ‘Pure olive oil’ or ‘Olive oil’ are usually a blend of refined olive oil and one of the above two categories of Virgin olive oil. Olive oil is a blend of virgin oil and refined oil, containing no more than 1.5% acidity. It commonly lacks a strong flavor. These standards are laid down by the International Olive Oil Council (IOOC) of which the vast majority of Olive Oil producers are members. The USA uses a different system, as usual. (Source - Wikipedia)

I like the stuff but I really never thought how they come up with it.

Basically, for cooking at home, I shop for Extra Virgin Olive Oil (First Cold Pressing). If you are substituting it for other oils in baking or frying, go for Virgin or Ordinary Virgin because they have less flavor. Hope this guide below helps! ------------ This oil is obtained only from the olive, the fruit of the olive tree, using solely mechanical or other physical means in conditions, particularly thermal conditions, which do not alter the oil in any way. It has not undergone any treatment other than washing, decanting, centrifuging and filtering. It excludes oils obtained by the use of solvents or re-esterification methods, and those mixed with oils from other sources. It can be qualified as a natural product, and virgin olive oil can have a designation of origin when it meets the specific characteristics associated with a particular region. Virgin olive oils can have the following designations and classifications depending on their organoleptic (taste and aroma) and analytic characteristics (the degree of acidity refers to the proportion of free fatty acids, not to the taste) Extra Virgin olive oil Virgin olive oil which has a free acidity, expressed as oleic acid, of not more than 0.8 grams per 100 grams (0.8%), and the other characteristics of which correspond to those fixed for this category. Extra Virgin olive oil accounts for less than 10% of oil in many producing countries. Used on salads, added at the table to soups and stews and for dipping. Virgin olive oil Virgin olive oil which has a free acidity, expressed as oleic acid, of not more than 2 grams per 100 grams (2.0%) and the other characteristics of which correspond to those fixed for this category in this standard. Ordinary Virgin Olive Oil Virgin olive oil which has a free acidity, expressed as oleic acid, of not more than 3.3 grams per 100 grams and the other characteristics of which correspond to those fixed for this category in this standard. Ordinary oil may still be fine for frying or where flavor is not wanted or needed. Lampante Virgin Olive oil Virgin olive oil not fit for consumption as it is, designated lampante virgin olive oil, is virgin olive oil which has a free acidity, expressed as oleic acid, of more than 3.3 grams per 100 grams and/or the organoleptic characteristics and other characteristics of which correspond to those fixed for this category in this standard. It is intended for refining or for technical use. The Trouble with Definitions. There are many governing bodies which attempt to define olive oil. The European Communities has a different definition than the International Olive Oil Council (IOOC). Most countries use the IOOC standards. The U.S. is one of the few major markets which has not adopted the IOOC definitions. Instead the USDA has a 1948 classification which uses terms such as "fancy" and "choice". Olive oil importers have effectively blocked passage of more meaningful labeling. Working with certified IOOC tasting panels, the California Olive Oil Council has attempted to rectify this by issuing a seal to oils which exceed IOOC Extra Virgin olive oil quality standards. Look for the COOC seal to find oils which surpass global standards. http://www.oliveoilsource.com/definitions.htm