Wonder Under is an amazing product. I use it often, so buy online since prices are usually better than offline stores, especially since WalMart stopped carrying sewing items.
I have used Wonder Under to recreate pictures that my children have drawn by tracing the picture, cutting the pieces out of fabric, fusing them to a piece of muslin and then doing a tight zig-zag or satin stitch around all edges. I finish the edges with Fray Check or a similar product and the life of the piece is endless. I have washed and dried many pieces I've made using this method. Check out my dish towel project - this is a typical Wonder Under application, as are the many potholders I've made. I've used Wonder Under to fuse photos printed on fabric for use with memory quilts as well. You can see another use in my bib project.
Some people use Wonder Under without stitching the edged; they simply fuse and let it go. I've not had good luck with that method. In order to make something that you can wear or use several times, items you are able to wash and dry, you really do need to stitch the edges. The only reason I use a fray reducer is that one project with printed photos did some unraveling on the edges. The fabric upon which the picture was printed was fairly fragile. Better safe than sorry with usable items. As mentioned above, I made hand towels using Wonder Under and have washed and dried them a lot with only minimal expected wear that is uniform and not limited to the fused fabric.
You can buy WonderUnder on line and off. There are other products similar to it, but I've not used them. I tend to stick with what I know most of the time. Now, go crazy and make some stuff!