Chartres was built on an old site used by a Druid cult to honor "the virgin to come," and early Christians reconsecrated it as a regional center of Marian devotion. Sometime after the Carolingians lost power Chartres acquired the "tunic" that Mary allegedly wore while giving birth to Jesus. Interestingly enough, the piece of cloth, currently on display in the cathedral inside a reliquary, was recently dated to about two thousand years of age.
The cathedral is a giant stone-and-glass Bible. All of the windows can be "read" and interpreted easily (if you know what you're looking for, that is). And the colors are unusually saturated, supplemented by glass that is thicker than usual. The flying buttresses are also unique among Gothic cathedrals in France; they are lighter and lacier rather than solid masses of stone.
The town of Chartres itself is actually tiny, as it always was, and had Mary's tunic not escaped a late eleventh-century fire, the current cathedral, which functioned primarily as a pilgrimage site, might never have been constructed. Even today it dominates the little town, which sits on a hill amid wheat fields about an hour from Paris by train.
Here are some photos of the exterior (including the flowers around the cathedral), interior (including stained glass and votive devotional candles), and surrounding area. Note that most of the windows feature "sponsors" at the very bottom in the form of images of a particular trade guild that paid for the window, such as shoemakers, tailors, or masons.