Magna Carta makes the law formal, being both part of government and an instrument of justice and protection for individuals. The signing of the charter committed both king and barons to the principles agreed. Much of Magna Carta refers back to the traditional law of the land. In particular principles dear to Alfred the Great re-emerge in the charter; government by consent, the right to trial and to be judged by ones peers, royal officials must know the law and administer it fairly and the king not being above the law.
With his fluency in Latin it is likely that Stephan Langton, the Archbishop of Canterbury had a working knowledge of established law and blended traditional law with provisions demanded by the barons. The provisions of the charter reveal the grievances of the barons. They were clearly incensed at the crown's arbitrary demands for money in order for estates to be inherited, the crown exploiting the situation where the heir was a minor, selling guardianships, marrying off heirs and widows with no real consent, taxation without consent or consultation and royal officials simply seizing goods from land and horses to corn and homes. The charter also reflects that the barons can't do these things to their tenants either.
The charter embodies many of the elements of freedom and democracy which is why its provisions occur in many countries constitutions including that of the USA. Amongst the provisions are freedom of the church without state interference in appointments, taxation only by consent, the state cannot force people to marry, free men have liberties, cities and towns have traditional rights, punishment in proportion to the offence and the persons wealth, no arbitrary seizure by the state of peoples homes or possessions, the right to trial, the right to witnesses, trial by peers under law, justice is not for sale and restrictions on royal monopolies and exemptions. These fundamental rights were often attacked by kings notably James I, Charles I and George III. Not surprisingly the attempted overthrow of consensus government represented by Magna Carta produced the English Civil War and the American War of Independence. Monarchs tended to view the colonies such as the Bahamas and Maryland as royal monopolies where the provisions of Magna Carta did not apply. Taxation without representation was one of the main factors in the American war of Independence.
The main provisions of Magna Carta are:
It is well worth reading the full text of Magna Carta which is available here on the British Library website in it's modern English translation and it doesn't take long to read. Pope Innocent III was enraged by Magna Carta and set it aside however the upholding of the charter's provisions soon became part of the coronation oath of English Kings. There were many disputes over the provisions of Magna Carta in the centuries to come with the ultimate test being under Charles I. The abiding result of this was that if the monarch tries to abolish the law he will be judged by it. The long Parliament established the principle that the king could not simply govern without parliament, parliament had to be called.
Stephan Langton almost certainly is responsible for the bulk of the charter, it was a great achievement.