If it varies with driving speed, look for an out-of-round tire. If it vibrates only when braking, you probably have a warped brake rotor.
It could be a shifted belt in one of the tires.
http://www.2carpros.com/topics/shakes1.htm
Wheels are out of balance
Perhaps front rotors are "out of round" and need to be either re-surfaced or replaced
Tune up
Engine miss at idle. Can be caused by vacuum leak.
Your power steering pump and or steering box may be about to give out on you.
Sounds like an engine misfire. Can be bad spark plug, plug wire, fuel injector, or bad coil on plug.
Loose or worn steering parts, suspension worn, tires out of balance.
balance your tires
Worn steering or suspension parts. This can be very dangerous, so have this repaired ASAP.
There are several causes of this but the most common we call a head shake. This is when the tires shake side to side, the cure is to insure the steering is tight and you add one or two hydraulic steering dampers.