Constitutional law is the study of the laws created by or arising from a constitution. A constitution is a formal document which states the foundational body of rules that provides a framework for the governance of a nation-state. It outlines the rights of the people, the form of government, and importantly, imposes limitations on that government.
The United States Constitution, for instance, calls for a government that has checks and balances in place, such that it cannot (or should not) be eroded by a few bad or corrupt leaders. The U.S. Constitution separates our federal government into three branches: the executive branch (the presidency); the legislative branch (congress); and the judicial branch (the courts). This division of power is often referred to as the "separation of powers." The constitution outlines the responsibilities of each such branch and the ways in which the failure by one branch to properly exercise its powers can be checked by the other branches.
Constitutional law also provides for a number of fundamental rights available to the citizens it governs, such as the right to free speech (1st Amendment) and rights of equal protection and due process of law (14th Amendment).
All fifty of the United States have state constitutions as well. While the states are free to enact whatever rules and rights they may wish for, no state may take away any rights which are provided for by the U.S. Constitution.
Constitutional law is a form of law which follows the American Constitution, a document originally drafted in 1783 by some of America's founding fathers. The Constitution of United States, a federal document regulated both civil and criminal laws as well as procedures, is regarded as the superior document in the United States government. Essentially, Congressional Amendments, specifically the Bill of Rights (the first ten constitutional amendments), are broken up into two categories: amendments which limit the power of the government (such as Amendment three which does not allow the government to quarter troops and amendment one's establishment clause which prevents the government from establishing a state sponsored religion) and amendments which extent power to the people (first amendment protections, the right to bear arms, etc.)