Mont Saint-Michel is an "island" off of the coast of Normandy in France. When most people hear Normandy, WWII is the only thing that come to mind. France is actually split into regions much like the United States is split into states. One of these regions happens to be Normandy, which is where we invaded and where Mont Saint-Michel happens to be located.
Mont Saint-Michel was first used in the sixth and seventh centuries as a place for the Romano-Bretons could have culture and power. Basically as a place of status, and was invaded by different groups for a couple hundred years. At the end of the eighth century, construction began for the first monestary. The monestary was firmly established during the eleventh century and turned into a prized posession by kings of France. The English tried to invade it many times unsuccessfully, which just added to the pride in the piece of land.
Finally, by the time of the French Reformation, there were barely any monks living at the monestary. So, to be efficient, the French turned it into a prison to hold people against the current regime. The prison was closed in the mid 1800s and became a national monument. It has since become a much visited spot for tourists to see culture as well as natural and man-made beauty.
So like I said, this was originally used as a monestary but the uses evolved over time. Which adds to the architecture that can be seen. The center-piece of the island is the cathedral/monestary at the top of the island. It has been restored through-out the years, but has kept the original charm. The cathedral is actually really cool because it has been added on to over the centries. So there is every type of architecture imaginable all in the same structure.
So besides the obvious monestary on the island, there are other things to be seen. As was mentioned, there is a pretty sweet prison on the island. The prison, though developed later, is underground. This adds a bit of a creepiness to the island to make it a little more interesting. There is also a town on the island where people actually still live and have shops for the tourists. And trust me, these people make money souly on tourism, so don't expect a good deal. However, there are some very impressive artists on the island worth the money.
Visiting Mont Saint-Michel is a must do while in France. The views from the mainland are just magnificant and worth your while. The architecture will impress anybody as will the friendliness of the townspeople.