Butter is churned cream of milk. An unctuous (usually) yellow solid fat, often spread on bread or used in the production of cakes, biscuits and pastries. Oleomargarine (margarine) is a butter substitute made from vegetable oils.
Until recently dairy farmers denounced oleomargarine as an inferior chemical substitute for butter. It is actually just as nutritious, and a lot cheaper. One tactic was a law that said margarine could not be colored yellow, so it came out looking like lard.