The quality of olive oil is classified by the level of acidity, taste, and aroma. Olive oils higher in acidity can be rectified, or treated with chemicals to lower the acidity, but are called refined, not virgin.
Olive oil has the same amount of calories that other oils contain -- 120 calories per tablespoon. But olive oil is highly unsaturated and has been suggested as a healthier alternative to more saturated fat or oils. Additionally, olive oil is a highly flavored oil, so you can use much less than oils with lighter flavors.