If the steering wheel vibration happens at low speed, it's a tire or wheel problem. Low-speed wobbles or vibrations are almost always caused by this. You might have a tire with a bad belt that causes it not to be round, or you might have a wheel that's bent due to a collision or some other trauma. Come to think of it, it could also be that the wheel is loose on the hub. Don’t laugh, I’ve seen it happen. Remember this is at low speeds, less than 30 mph or so.
If your steering wheel vibration is at a higher speed, such as 55 to 60 mph, it's almost always tire balance. Vibrations mean that something is out of balance. Things that rotate, such as tires, crankshafts, and axles, all have a certain resonance at a given speed. Engineers deal with this resonance by balancing the component by adding or removing weight at strategic points. This helps prevent a vibration at a given RPM.
You might have noticed that if you go at a slower or faster speed, the vibration goes away. This is due to the resonance effect I'm talking about. There's a given speed at which a vibration will show up. Hit that speed and the vibration will be pronounced. Go faster or slower, and no more vibration.