The American constitutional system includes a notion known as the Separation of Powers. In this system, several branches of government are created and power is shared between them. At the same time, the powers of one branch can be challenged by another branch. This is what the system of checks and balances is all about.
There are three branches in the United States government as established by the Constitution. First, the Legislative branch makes the law. Second, the Executive branch executes the law. Last, the Judicial branch interprets the law. Each branch has an effect on the other.

Legislative Branch

• Checks on the Executive

o Impeachment power (House)
o Trial of impeachments (Senate)
o Selection of the President (House) and Vice President (Senate) in the case of no majority of electoral votes
o May override Presidential vetoes
o Senate approves departmental appointments
o Senate approves treaties and ambassadors
o Approval of replacement Vice President
o Power to declare war
o Power to enact taxes and allocate funds
o President must, from time-to-time, deliver a State of the Union address

• Checks on the Judiciary

o Senate approves federal judges
o Impeachment power (House)
o Trial of impeachments (Senate)
o Power to initiate constitutional amendments
o Power to set courts inferior to the Supreme Court
o Power to set jurisdiction of courts
o Power to alter the size of the Supreme Court

• Checks on the Legislature - because it is bicameral, the Legislative branch has a degree of self-checking.
o Bills must be passed by both houses of Congress
o House must originate revenue bills
o Neither house may adjourn for more than three days without the consent of the other house
o All journals are to be published

Executive Branch

• Checks on the Legislature

o Veto power
o Vice President is President of the Senate
o Commander in chief of the military
o Recess appointments
o Emergency calling into session of one or both houses of Congress
o May force adjournment when both houses cannot agree on adjournment
o Compensation cannot be diminished

• Checks on the Judiciary

o Power to appoint judges
o Pardon power

• Checks on the Executive

o Vice President and Cabinet can vote that the President is unable to discharge his duties
Judicial Branch
• Checks on the Legislature

o Judicial review
o Seats are held on good behavior
o Compensation cannot be diminished
• Checks on the Executive
o Judicial review
o Chief Justice sits as President of the Senate during presidential impeachment