The Medicine Wheel is a religious symbol for Native American religions, just as the cross is for Christianity, the pentagram for Neo-paganism, or the crescent moon for Islam. In Christianity, you might see the cross as a necklace, as a marker on top of a steeple to denote a church, or on a sign to signify a church. The symbol is the same, although the uses are different. The same is true for the Medicine Wheel. There is the great Medicine Wheel in Wyoming, which might have been an astronomical aid. The ancient Native village of Cahokia (St. Louis) had a medicine wheel, which possibly had the same use. Medicine Wheels are used as ornaments (earrings), or as aids in prayer (porcupine quill medicine wheels).
In most Native religious beliefs, the Medicine Wheel is a symbol for the four directions, and the spirits who guard those directions.