What is the Magna Carta and why is it so important?

It is the most famous document in history - the Magna Carta - and it was signed, stamped and sealed 800 years ago today. This humble piece of parchment is celebrated for creating the very concept of human rights, the idea that "all men are equal".
But what exactly is the Magna Carta? And who was it really written for?
  See BBC video here.