Extra Virgin Olive Oil = Unrefined oil from olives. The olives are pressed and the oil goes directly into a bottle to be sold. The oil is "impure" so tastes strongly of olives.

Olive oil = refined oil from olives. Similar to canola oil, sunflower oil, vegetable oil, etc. "Impurities" have been removed so the oil is mostly colourless and tasteless.

A lot of recipes use the flavour of the unrefined "extra virgin" olive oil and would taste off if the refined olive oil (or any other type of oil) was used.