What are the similarities and differences between butter and margarine?

Butter consists mainly of fat derived from milk whereas margarine, or imitation butter, is based on vegetable fat, or a combination of vegetable and milk fat. Butter and margarine can be used in place of each other. There are, however, 2 basic kinds of margarine: 1) shelf stable margarine, an inexpensive margarine that is easy to store and use, and 2) margarine that must be kept refrigerated. The latter is produced from unsaturated fatty acids such as sunflower oil, olive oil, cottonseed oil or canola oil and is healthier because it does not accumulate in the body and in fact, even helps reduce cholesterol.