COMMON LAW
Before the Norman conquest in 1066, justice was administered by local courts, under the rule of the bishop and the sheriff, who represented the religious and the civil power.
In 1154, Henry II became the first Plantagenet king. He made a lot of changes, one of the most important is the institution of common law, creating a unified system of law "common" to the country mixing all the local custom into a whole national system of rules, and eliminating arbitrary judgements.
Henry II used to send judges from the central court to hear the various disputes throughout the country. His judges would resolve disputes according to the customs. The king's judges would then return to London and often discuss their cases and the decisions they made with the other judges. These decisions would be recorded and filed, so they became a common law, instead of the pre-Norman system of local customs and law which were different in each place.
Henry II's creation of a powerful and unified court system, which surpassed the power of the church provoked a conflict with the church, in particular with Thomas Becket, the Archbishop of Canterbury. Eventually, Becket was murdered inside Canterbury Cathedral by four knights who believed that the murder would please king Henry.
The common law constitutes the basis of the legal systems of: England and Wales, Northern Ireland, Ireland, federal law in the United States.