I had the same question a couple of months ago and this is what I came up with. There are two kinds, Spacers and Adapters. Spacers are just the thin pieces of metal sheet that allow one to use rims that might not otherwise fit due to calipers etc. They range from 1/8" to 1/4" or a little more and are generally warned against unless you get longer lug studs, especially in the front where it is possible to strip studs. Although some mentioned running them for years with no problems.
Secondly there are adapters. Thicker, sometimes billet material with built in lug bolts. Bolt the adapter to the hub, then the wheel to the adapter. Generally used to allow the use of wheels of a different lug pattern, or as you want to do, to widen the track with original wheels, or wheels of a different back spacing.
I have the later, which I purchased on E=bay from "sixlugsam". I used 1.5" adapters front and rear and really like the look. I still have my original wheels and tires. I have only had the spacers on for about 2 months, but no problems so far, and I don't really expect any. One thing that I plan on doing is putting longer studs in the front for some piece of mind, as the lug bolt is 1/4" shorter in the front than the rear. In the rear the lug bolt is flush with the lug nut, on the front it is not. I'll do that as soon as I can get some info on how it is done.
The general consenses on adapters was that they are very strong, might cause some premature wear on bushing and bearings.
Again, this is just what I found in searching multiple forums/sites, and my own personal and VERY limited experience. Later