From CrashCourse
In theory, the Legislative Branch, the Executive Branch, and the Judicial Brach are designed to keep each other in check, and to keep any branch from becoming too powerful.
Watch video (8:31): https://www.youtube.com/watch?v=0bf3CwYCxXw&list=PL8dPuuaLjXtOfse2ncvffeelTrqvhrz8H&index=3