A plug-in is a single filter or a set of filters with it's own interface that can be installed in image-editing applications such as Photoshop and Photoshop Elements. A plug-in is like a program within a program. Photoshop-compatible plugins are developed and sold by 3rd parties in most cases, not by Adobe.

Plug-ins can add functionality i.e. noise reduction, sharpening, enlarging images, etc. or can enhance image quality or are often used for quickly creating special effects with a few mouse clicks that would be impossible or difficult to achieve with your image editor alone. They are integrated into the application and are found under the Filters menu.

Always read the requirements carefully or download the trial version before you buy to make sure the plugin is compatible with Elements or other editor such as PaintShop Pro and Gimp.