Considering that olive oil has been used for thousands of years, it really is a wonder product. The health benefits of olive oil have been well demonstrated... primarily due to both its high content of monounsaturated fatty acids and its high content of antioxidative substances.
Extra virgin is the first oil derived when the olives are pressed. It's best used for salad dressings and sauces. There are several varieties of olive oil. The common press used for cooking is virgin and pure... virgin is taken from the second pressing and pure is filtered and somewhat processed but retains many health benefits and imparts a buttery flavor to cooked dishes. Additionally, extra virgin has a high smoke point... around 410 degrees F, which makes it on par with most other vegetable oils and is excellent for dishes requiring higher intitial heat. I use it in Ratatouille, since the vegetables can be browned without losing firmness... I use it almost exclusively...(except I always use lard in biscuits, a scone like breakfast bread common here in the U.S.)...