Pepakura (or "pep") is a Japanese word which means "papercraft". We use it to refer to paper 3d models, much like origami, that are created using a program called "Pepakura Designer," made by [Tama Soft]. With this program, a 3d model file can be imported and then unfolded into patterns that can be then printed out onto paper, cut out, and then folded and glued into a paper 3d model.
The original 3d model file, once imported into the Pepakura Designer program, and unfolded, is saved and distributed to the masses as a ".PDO" file (or as a "pep file". Unfolding a 3d file correctly can be tedious and difficult, which is why PDO files are so popular; all the work has been done for you (except the printing, cutting, folding, and gluing!).
Pepakura Designer is availible as a free demo that will not allow you to save PDO files, or you may buy the full version relatively inexpensively and begin distributing your own PDO files for others to use.
With the Pepakura Designer installed, and a PDO file, you're almost ready to print. All you need to do is scale the model, reshuffle the patterns on the pages, and get some decent paper ready in your printer.
Some people have problems finding the correct scale, but there is a short set of instructions availible here on the wikia that should help you in Adjusting the Sizes It is also strongly recommended that you read the Pepakura Tutorial, which is a great place to learn how to do it correctly, with pictures to help you.