Magna Carta was a charter forced on King John of England on 15 June 1215 by feudal lords who were angry about the arbitrary power the king exercised over them. With Magna Carta, English monarchs began to be bound by law.
The immediate beneficiaries of the rights spelled out in the document were comparatively few. It specifically granted various rights to nobles, to the Church, and to free men. At that time most people in England were serfs.
The right of trial by one’s peers was established by Magna Carta. By the end of the 13th century that right was indeed extended to everyone in England.
Much of Magna Carta dealt with medieval property and inheritance matters. It also covered specific issues relating to river navigation, weights and measures, and use of forests. But the document set important lasting precedents.
Laws tended to be unwritten*. So copies of Magna Carta were made and placed in repositories around the country after King John agreed to it. It would be difficult for the monarch to pretend that the agreement did not exist.
In the 1600s the forces backing Parliament used Magna Carta as a rallying point in their struggle against royal power.
American colonists opposing unfair taxation and arbitrary rule cited Magna Carta in their protests against the British government.
The listing of specific rights in Magna Carta established a pattern which was followed by the 1689 English Bill of Rights, the 1776 US Declaration of Independence, the French 1789 Declaration of the Rights of Man and the Citizen, the 1791 US Bill of Rights, the UN’s 1948 Universal Declaration of Human Rights, and the 1982 Canadian Charter of Rights and Freedoms.
* In fact, the UK in the 21st century still does not have a written constitution.
Magna Carta is featured in this American Revolution era version of the Massachusetts state seal. 
One of the eight scenes on the great bronze doors of the US Supreme Court building depicts King John and one of the barons.
The Royal Mail is issuing a set of stamps to mark the anniversary. Here is one of them in detail.
The British Library has produced a video on Magna Carta narrated by Terry Jones of Monty Python.
The New York Times has a ten question quiz on Magna Carta.
See how well you do on it.
As Magna Carta Turns 800, How Firm Is Your Grasp on Medieval Law?
For further reading:
• Official Magna Carta 800th anniversary site
• Magna Carta: A turning point in English history
• The Great Charter of Liberty
• Magna Carta was forged from royal failure
• Text of Magna Carta (English with annotations and index)
• Text of Magna Carta (Bilingual with the original Latin)
TL;DR Civics 
Interesting article and the influence of Magna Carta can still be seen today in some of the individual state constitutions as well as the Bill of Rights.
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