The wheel hub bearing on your vehicle holds the tire and wheel in place. It allows them to spin and is an essential safety and operating component of the vehicle. Wheel hub bearings, specifically, refer to sealed units that are purchased as on part rather than in various parts, which is more common on older vehicles. A wheel hub bearing may be referred to as a "hub kit" or "hub assembly."
When a bearing is beginning to go bad, which often happens at higher mileage (120,000+), you will hear a low growling or humming noise as you drive or coast the vehicle. On many modern vehicles, the ABS warning light will come on as the hub bearings go out.
To test the bearings, lift one side or one end of the vehicle off the ground using a jack and stands. Grabbing the tire at the top and bottom (12 and 6 o'clock), move it in and out, pulling on both positions simultaneously. If there is a lot of movement or free play, your bearings are likely bad. Do this for all four wheels.
If you're still unsure, drive to your local shop and ask them to check it. Most will do so for free.
Replacing wheel hub bearings is a task that most competent home mechanics can do themselves. They are an extremely important part of your vehicle's safety, however, so when in doubt, have a pro do it.
Each vehicle is different, but most of the replacement procedure for hub bearings is the same. You'll need a replacement assembly (it's recommended that you do both front or both rear or all four at the same time) for each wheel to be done, a way to safely lift the vehicle at least one corner at a time (or all at once). You'll need several standard hand and shop tools like breaker bars, a Spindle socket, ratchet, screwdriver, rubber mallet, slide hammer, c-clamp, and torque wrench.
It's recommended you take photos throughout in order to document where parts go when replacing or returning them.
Remove the tire and wheel, brake assemblies (out of the way, complete removal is not required usually), and spindle nut. The wheel bearing assembly bolts are usually behind the knuckle. Loosen and remove those. Using a slide hammer on the lug studs, and use it to remove the hub bearing from the knuckle.
Clean the rust and corrosion off the knuckle and backing plate carefully. Replace and install the new bearing and put everything back together in reverse order. Note the torque requirement for the hub bolts and spindle.