From some quick googling, it looks like you can get oil out of the olives easier by heating or otherwise refining them with chemicals in some way, but this affects the taste and nutrition, so virgin olive oil is olive oil you get only mechanically ie by pressing/crushing it, extra virgin olive oil is a high quality grade of virgin olive oil, and regular olive oil can be made by heating or chemical refining with a bit of the virgin olive oil mixed back in to give it some of the olive flavor back.