How Federalism Impacts State Marijuana Laws

What is federalism, and how does it impact state Marijuana Laws?

Do federal laws reign supreme over state laws, or do state laws reign supreme over federal laws? What part of the United States Constitution holds the key to this answer? If marijuana were removed from the federal Controlled Substances Act, what implications would this have for states that have legalized marijuana? What about states that have yet to legalize recreational or medicinal marijuana? What impact would this have on those states?