The following classifications are used for imported oils:
… Extra Virgin: This is the highest quality-with the most taste, stability and expense. It's the most recommended of all olive oils with less than one percent natural acid, and the greatest range of flavors. 
… Virgin: Contains between 1.0 and 3.3 percent acid, which adversely affects the taste. It is, however, still a good oil. When the budget is tight, use this for cooking and extra virgin for salads. This is hard to find in the U.S. 
… Pure: This oil has been neutralized with chemicals to higher than 3.3 percent acid. It's the lowest and cheapest grade with the least flavor. Because this oil has little or no taste, some extra virgin oil is added back (520%) for flavor. 
The following are used for U.S. Olive oils, which are rated differently than imported oil. They are also hard to find: 
… Virgin: The highest grade and first pressing, but this does not have the variations of tastes like the imported oils. It is the most expensive of the U.S. types. 
… Refined: This is the second pressing, with chemicals added to lower acidity. Because of poor taste and lower quality, it is not recommended. 
… Refined Olive-Residue Oil: Something you might use for the squeaky cellar door. This is made from all the leftover olives, including the pits. The oil is extracted and neutralized using chemical solvents.

If you must heat oil, always cook at the lowest temperature for the least amount of time possible. Adding a small amount of water with the oil will prevent too high a temperature. (Use butter whenever possible for cooking.) Be sure to refrigerate all oils after opening, despite what it says on the label, unless it's used within a month. The oil will thicken slightly in the refrigerator, but will liquify quickly by holding the bottle under warm water. Although olive oil's stability allows it to last longer than most oils, all oils eventually become rancid. This is not only distasteful, but has an adverse effect on the body.