A fairing is any kind of shell placed over part of the motorcycle to guard against grime, improve aerodynamics, make for a comfortable ride and generally make your ride look boss. Usually we think of fairings in relation to handlebars and windscreens, but some motorcycles add varying degrees of wheel fairings, too, either pre- or post-market.
Most wheel fairings are designed for style. If you're into the look, that's the reason to buy. But for a highway cruiser, some fairings can increase your gas mileage.
There are a thousand bike designs and another thousand fairings. If you're buying used or specialty fairings, just make sure the additions match your model.