Separation of Powers means that the government is divided into three branches, the Legislative (Congress), Executive (the President), and Judicial (federal courts). By doing this, the government is making sure that they don't gain too much power.
For Example: The House and Senate may pass a law that the President does not like. He can veto the law. To pass the law, the law needs 2/3 vote of both House and Senate.
Federalism divides the power between a main central government and smaller, local governments. This is to make sure that the central government is powerful enough to hold authority without holding too much power that it will threaten the other states and citizens.
For Example: Freeways are owned and operated by the states, but their rules and guidelines are set by the federal government.