The constitution of a country (or a state) is a special type of law document that tells how its government is supposed to work. It tells how the country's leaders are to be chosen and how long they get to stay in office, how new laws are made and old laws are to be changed or removed based on law, what kind of people are allowed to vote and what other rights they are guaranteed, and how the constitution can be changed.
Limits are put on the Government in how much power they have within the Constitution (see Rule of Law ). On the other hand, countries with repressive or corrupt governments frequently do not stick to their constitutions, or have bad constitutions without giving freedom to citizens and others. This can be known as dictatorship or simply "bending the rules". A Constitution is often a way of uniting within a Federation.
The UK's constitution is not written in one single document like many other countries' are. In fact, the UK's constitution is not completely written down at all. Some of it can be found in writing, starting with the Magna Carta of 1215 and the Bill of Rights Act 1689 and including more modern Acts of Parliament. Other parts of it are considered common law and are made up of the decisions of judges over many hundreds of years in a system called legal or judicial precedence. Because of this, some people say that the United Kingdom has a de facto or "unwritten" constitution.
The United States in 1787 began a trend in the writing of constitutions. The United States Constitution is also the shortest that people are still using, and it has been changed (amended) many times over the years. It was made after the colonists won their independence from Britain. At first they had the Articles of Confederation but the Articles were replaced with today's Constitution.
The Indian constitution of 1950 is the longest ever written constitution in the world. It has 448 Articles and 12 Schedules in it, with 5 appendices and 98 amendments.