A wheel cover is the technical term for the ornamental
covering of an automobile's bare wheel
. They can be pried off and pushed back on again with little effort. Older wheel covers are made of metal, but newer ones tend to be of painted plastic.
Wheel covers are usually much cheaper than rims, and are generally found on your more inexpensive rides.
Repeated hard cornering often results in the ejection of the outside front wheel cover, causing our highways to be littered with large mandalas heralding Ford and Toyota for the pedestrian's amusement.
Note: The term hubcap is often used for these objects; wheel cover is a more technical term, in usage in the Pacific Northwest, although I've heard it in the Northeast as well.