A constitution is a system for governance that establishes the rules and principles of an organisation, or more loosely, a group of people.
In the case of nations, it defines the fundamental political principles, and establishes the structure, procedures, powers and duties, of a government. Most national constitutions also guarantee certain rights to the people.
National constitutions seek to regulate the relationship between institutions of the state, in a basic sense the relationship between the executive, legislature and the judiciary, but also the relationship of institutions within those branches. For example, executive branches can be divided into a head of government, government departments/ministries, executive agencies and a civil service/bureaucracy. Most constitutions also attempt to define the relationship between individuals and the state, and to establish the broad rights of individual citizens. It is thus the most basic law of a territory from which all the other laws and rules are hierarchically derived; in some territories it is in fact called "Basic Law."Generally, all constitutions give specific powers to an organization on the condition that it abides by this constitution or charter limitation.
This question was asked on the wiki: WikiConstitiution.