Guernsey is one of the major islands of the Channel Islands (also known in some languages as the Norman Islands). They've been inhabited since prehistoric times, with greater settlement picking up during the Middle Ages. Originally, they were a part of the Duchy of Normandy, before being ceded to the English monarchy in the 13th century, and later becoming an extension of the British monarchy.
Guernsey belongs under the autonomous unit known as the Bailwick of Guernsey, along with the islands of Alderney and Sark, and is the biggest of the three. The Bailwick of Guernsey is one of the three existing crown dependencies of the United Kingdom. All three are located on particular islands or archipelagos in the seas surrounding Great Britain, with a special autonomous status of their own. Though they've been politically British for a long time, culturally and linguistically, they show a mixing of past influences.
Jersey - The other large entity of the Channel Islands, with its own crown dependency.
The Isle of Man - The crown dependency in the Irish Sea.