Overview
Separation of powers is a doctrine of constitutional law under which the three branches of government (executive, legislative, and judicial) are kept separate. This is also known as the system of checks and balances, because each branch is given certain powers so as to check and balance the other branches.
Each branch has separate powers, and generally each branch is not allowed to exercise the powers of the other branches.
The Legislative Branch exercises congressional power, the Executive Branch exercises executive power, and the Judicial Branch exercises judicial review.
Further Reading
For more on separation of powers, see this Florida State University Law Review article, this Cornell Law Review article, and this Northwestern University Law Review article.