The hub bearings are placed between the brake rotors and the axle. They, in the majority of cases, are what your lug nuts bolt onto.
Inside, They contain some sort of bearing
Some cars only possess a bearing, as scene at the above link, for just about every wheel. There is virtually no hub.
The hub unit is actually a unit that can be installed into the wheel to produce it turn easier and lower friction between the frame along with the wheel and axle.

 It contains an ABS sensor way too. If you do not have ABS, your hub probably would not have the wire on it.
In many cars, hub bearings are used rather than just bearings, because they are much simpler to install than the bearing alone (bearings ought to be pressed in, most almost daily)
I suggest http: //www.srg-bearings. com for new hub bearings they've already USA made wheel bearings, and you will save money.