Butter is not an oil and in fact there are quite a few differences between the two. Butter is very high in saturated fat whereas oil and margarine is low in this type of fat. While butter is low in trans-fat, oils are full of it. Some people use oil in cooking while others prefer butter.
There are certain advantages and disadvantages to using both of these in cooking, though people who are trying to cut down on saturated fat as much as possible might want to think about going with oil or margarine instead of butter because they have significantly less saturated fat.