We all first learned about the Magna Carta in middle school (ok, fine, high school. Ok, fine, earlier today on todayinhistory.com.) The name comes from the Latin Magna, meaning great, and Carta, meaning cart, because like a great cart, it pulled the people forward and gave them lots of rights. Issued in June of 1215 by King John, it is the most famous document in British constitutional history. Of course, it didn't start out that way. It had to climb its way up to the top, past King Arthur's "Check out this sword I pulled out of this rock," and King Aethelwulf's "A Treatise On People We Should Kick Out Of England, Ok, You Got Me, I Really Just Mean The Jews." But, like Angelina Jolie, the Magna Carta knew once it made it to the top, it would stay there forever, and everyone would forget about its humble beginnings, such as the time it made out with its brother on NATIONAL TELEVISION I MEAN ARE YOU KIDDING ME?!?!?
By June of 1215, things weren't going well between King John and the barons. Barons were rich people who leased land from the King to rent out to semi-rich people. Barons could even mint their own money. Some people think the barons really needed to stop complaining and just thank the Lord they weren't serfs -- those people know from problems -- to which the barons responded, "Yeah? Well I'm going to kill your whole family." That ended that conversation.
The Barons and King John continued to clash over complex issues of economics, authority, and royal privilege. The barons were like, "Stop appointing advisors from outside the baronial ranks, and King John was like, "Go to hell." So the barons brought him a list of demands that turned into what we now know as the Magna Carta. The Magna Carta required the King to accept that he was not above the law, even if he stood on his tippie toes. It protected the subjects' rights whether they were free or fettered. Many people, especially those who write articles on this subject for Wikipedia, would say the Magna Carta is the most influential and important foundation of constitutional law. Before, the kings of England could just do whatever they wanted all willy-nilly. The Magna Cart limited the king's power, so really he could only be willy-nilly sometimes.
Of course, what the barons first presented to King John was just a draft. It had to be edited. Sexed up. And could there be a part for a sassy butler? But soon King John put his seal on the final version, so I think that last one is the official Magna Carta. Or the writ of habeas corpus, which is also somehow related to this. Or the reform bill. I will look into this.