"Decoupage is artistic sleight of hand. What appears to be a painted design is actually paper, glued in place. What looks like lacquer is just a few coats of clear varnish, and what begins as an unremarkable piece of furniture becomes a bold, modern design statement. We've gathered our best ideas for you to use in your home or to make as original, custom gifts for friends." So says Martha Stewart's web site (http://www.marthastewart.com/print/274817).
Regardless of what one may or may not think about Martha in terms of the financial shenanigans that sent her to prison for a while, it's hard not to admire her for helping to explain crafts techniques in easily understood ways. I used to enjoy watching her on TV for that very reason.
My mother was the person who first introduced me to decoupage, back in the 60s when I was a kid. She took some wood plaques (which she made from lumber from the local lumber yard) and laboriously sanded them and prepared them to receive the glued-on artistic images, which she had clipped from magazines and similar sources. There was no medium designed specifically for decoupage, as I recall, but she still managed to create some very nice plaques with which she decorated our home.
These days, it's possible to buy prepared plaques and panels (wood, MDF, metal, bamboo, etc.). No need to laboriously sand and prepare lumber from the local lumber yard. (One support that particularly attracts me is known as Duho. But there are numerous other competing options.) As for the decoupage medium, art supply stores and crafts shops sell Mod Podge and Aleene's specifically for the purpose of making decoupage.
Decoupage can be used to decorate all kinds of products, including furniture, papier mache boxes, bound Moleskine journals and much more. My primary interest, however, is in using the technique to add digitally printed images (made at a local store which offers laser photocopies or digital laser prints) to the aforementioned plaques and panels.
There are also digital flatbed printers such as the marvelous Durst Rho, capable of printing directly on plaques and panels with no need to mount or transfer images to those supports, but the cost of such units can be much higher than most mere mortals can afford, so if you go that route, you will need to find a company which can print your files. For many situations, decoupage can be a much more accessible option.
For the purpose of making salable products which need no framing or matting, decoupage can sometimes be a great option. The fact that they need not be framed is useful because one does not have to worry about glass or plexiglass "glazing" materials breaking during shipment. I hope to make such pieces to be sold via the nearest eBay consignment shop, to bring in a little bit of extra money.