It is important to consider what width you will choose for your new wheels, compared to the original equipment items fitted to your vehicle. The wheel width must be looked at in conjunction with wheel offset when determining how the wheels will fit under your vehicle's guards and also what tyres you can fit to them.
Running narrower wheels tends to improve vehicle ride comfort and cornering in the wet at the expense of traction in the dry, due to the narrower tyres that will be required. Conversely, choosing wider wheels allows you to fit wider tyres, which will generally improve dry traction (and off-road performance for 4WD vehicles) at the expense of wet cornering and some ride comfort.
As you might have guessed, wheel width directly affects the width of the tyres you should have fitted to your vehicle. Selecting wider wheels with narrower tyres may result in more vibration and a reduction in ride comfort, due to the sidewalls of the tyre being under increased tension.
On the other side of the coin, selecting wheels that are narrower than the tyres may result in poor handling as the tyre distorts sideways more when cornering. As a result, it is vital to get the wheel width correct right from the start to avoid running into these problems.