Plug-ins offer additional features to the standard program. Photoshop supports third party filters that can be added to the plug-ins folder to increase the program’s flexibility.
You can buy plug-ins such as Corel KTP Collection, AutoFX Photo/Graphic Edges and Alien Skin’s Eye Candy 4000 off the shelf. Many can be downloaded from the Web. I’ve included a tiny selection of interesting ones over the next few pages.