A constitution is a set of rules that are the ultimate source of legal authority for the state. The rules of the constitution identify the major institutions of the state, and govern the relationship between the state and the individual citizen.
Most national constitutions are explicitly codified, with the exception of the United Kingdom, New Zealand, and Israel.
Constitutions identify the principal institutions of the state: the executive, the legislature, and the judiciary.
Constitutions may be republican or monarchical. In a republic, the Head of State will be directly elected by the people. Under a monarchical constitution the position of Head of State remains with the current head of the monarchy, although in practice the national parliament may be headed by a Prime Minister.
In a federal state, the constitution ensures that the balance of power is diffused between powers that may be exercised by the central federal government and its constituent parts (usually states). Conflicts between federal and state concerns are resolved by referring to the constitution. A unitary state is when one Parliament has ultimate power over all of its constituent states, an example being the United Kingdom.
Finally, constitutions may be supreme or subordinate. A supreme constitution is considered sovereign, and not subject to change from any superior laws. In the case of former colonies, their constitutions may be subordinate, having been drafted and introduced by an external sovereign power, and in theory able to be repealed by that power.
The process for amending constitutions is generally quite difficult, often requiring majority votes from all parliamentary houses. The public may also have a say in constitutional amendments if the issue is put to them in a referendum. Specific percentages in order for the amendment to succeed depend on the nation in question.