Wheel spacers are round objects with holes that are used inbetween your wheels and your wheel mounting surface (Usually a disc brake or drum) to widen your track width. They are typically 1/8" to 3/8" thick. I have seen them used on only one side to adjust for an improperly centered axle.
Wheel adapters are often referred to as wheel spacers as well, but have the benefit of being able to space your wheels out even further than a wheel spacer, typically from 1" to 3" per side. They are also able to adapt your bolt pattern to a diiferent one, useful for matching up different axles. The farther you space out your wheels, the more added stress to put on the wheel bearings.
Opinions vary on wheel adapters. Some people swear by them. They serve their purpose, but
in my opinion, the added wheel bearing stress and the fact that you can't put your eyes on the inner set of lug nuts make them a less than ideal solution, and a temporary band-aid at best. Add to that the fact that wheel adapters and their lugs are typically constructed of soft aluminum to keep the weight down, which means that they will quickly become marred through regular use. I am not a fan of them at all.
Again,
in my opinion, wheel adapters do not belong on any serious off-road vehicle.
One of the first things I did to my explorer when I got it was put 1.5" thick, 5 on 4.5" wheel adapters front and rear to allow me to run 17" Cobra R wheels (which have a much deeper backspacing than the stock explorer wheels). I broke several of the aluminum studs simply by tightening down the lug nuts. I also lost a wheel on at least two different occassions due to the inner set of lug nuts loosening. I was 16 then, so probably they weren't torqued correctly, but the danger was still there.
![Image](http://demon4x4.org/2008/Mudd/Pics/Pics6.gif)