The hub bearings are located between the brake rotors and the axle. They, in most cases, are what your lug nuts bolt onto.
Inside, They contain a bearing Some cars only have a bearing, as scene at the above link, for each wheel. There is no hub.
The hub unit is basically a unit that can be installed into the wheel to make it turn easier and greatly reduce friction between the frame and the wheel and axle.
A good example of a hub assembly is at contains an ABS sensor too. If you do not have ABS, your hub would not have the wire on it.
In many cars, hub bearings are used instead of just bearings, because they are much easier to install than the bearing alone (bearings need to be pressed in, most of the time)
Rear wheel drive cars have serviceable and replaceable wheel bearings inside an easily removable front hub for re-greasing or replacement. The rear bearings in a rear wheel driven car are bolted inside a retainer flange at the end of each rear axle.