Canterbury is a major cathedral city in the the English county of Kent. It's name comes from the combined Jute and Anglo-Saxon name Cantwaraburh, which means "Kent peoples stronghold".
In the 6th century Pope Gregory the Great sent St Augustine (eventually St Augustine of Canterbury) to the city to convert Aethelred, the King of Kent, to Christianity. This was successful and Augustine set up his episcopal see in the city building his cathedral on the site of an earlier Saxon one in 602 AD. Therefore Augustine became the first Archbishop of Canterbury.
With the spread of Christianity throughout the country, Canterbury became very important and very wealthy because of trade routes, pilgrimages and specialized trades such as pottery, leather-work and textiles. It even had a city mint for the creation of it's own coinage. Then in 672 AD it's significance in England was sealed forever. The Council of Hertford, a meeting of the most important churchmen in the land, decided that the Canterbury see would have authority over the entire English church, making the cathedral the most important in the country and the Archbishop the most senior churchman bar the Pope. From this point on, in matters of religion, the Archbishop of Canterbury's word was law and exceeded only by the Vatican. One of the most famous Archbishops is Thomas Becket who was murdered in the cathedral by a group of Henry II's knights in 1170. He was canonized soon after by the Vatican.
Even after the reformation, the seniority of the Archbishop was kept and Canterbury maintained it's religious importance. Although no longer answerable to the Pope, the Archbishop is the highest member of the Church of England bar the monarch.