What is the Unwritten Constitution?

These are things we do that's not printed in the Constitution such as political parties, cabinet, judicial review, nominating conventions.

The "Unwritten Constitution" refers to the ideas and processes that are accepted as a needed part of American government, regardless of the fact that they are not actually in the Constitution. These ideas and processes came about through the custom and precedent. Many aspects of the unwritten Constitution are so ingrained into our system that many do not even realize that they are not laws or provisions of the Constitution.

Example #1 The President's Cabinet
George Washington was the first to have a cabinet of advisors to aid him in making decisions for the nation. Today, the president's cabinet consists of the secretaries/heads of the 14 major departments of the executive branch (a few examples include:  The Attorney Generalof the Justice Department, The Secretary of Defense of the Defense Department and The Secretary of State of the State Department).

Example #2 Political Parties
Political parties are as old as the Constitution itself, however the document makes no rules to govern them. The first political parties arose from the debates over Constitutional ratification (the Federalists Party lead by Alexander Hamilton and the Democratic-Republicansheaded by Thomas Jefferson). It quickly became clear that political parties were to be the major forces in shaping American politics and by necessity rules and laws were established for their regulation, with no basis in the Constitution.

Example #3 Congressional Committees
Congressional committees have been referred to as the "backbone of Congress". However, the system by which the majority of the work in Congress is accomplished is not defined in the Constitution. The work of Congress soon became so vast, that it became essential to the governing of the nation, to divide the work of legislating into specialized committees. These committees in both the House and the Senate serve to first consider specific legislation on topics such as the military (armed services committee), foreign policy (foreign relations committee) or spending (appropriations committee) before passing bills onto the entire House or Senate for consideration.

Example #4 Judicial Review
The most important power of the Supreme Court, that of Judicial Review or the ability to declare laws unconstitutional, is not in the Constitution. The power of Judicial Review was a precedent set in the 1803 Marbury v. Madison decision. Regardless, it has become one of the most fundamental concepts in American  government. It also serves a check and balance on the laws passed by Congress and the actions and treaties of the President.