Prometheus and Epimetheus were both titans, the immortals that came before the gods. Prometheus and Epimetheus were brothers. Prometheus's name means "forethought" meaning he could see ahead what the consequences of his actions would be. This is why he chose to help Zeus and the other gods during the war between the titans and the gods. Prometheus is considered the wisest of the titans. He was the protector of men. In some myths he and his brother, Epimetheus, created man. In all myths Prometheus was the immortal that brought fire to man, thereby angering Zeus and he was punished. Epimetheus means 'afterthought' so he always acts before thinking and was considered very stupid. He only fought on the right side because he did as Prometheus instructed him. Epimetheus accepted Pandora as a gift from Zeus. She was given to him as his wife and it was she that brought evil into the world.
So, to summarize, Prometheus is the opposite of Epimetheus. Prometheus was wise and could look ahead to what consequences his actions would bring; while Epimetheus, it seems, blundered along and acted before putting thought into the consequence.