A plugin is a software add-on that is installed onto a program, enabling it to perform additional features. For example, Internet browsers allow users to install plug-ins into the browser to give that browser features not found in the default installation.
Depending on the plug-in that is required on a website depends on from where the plug-in is going to be downloaded. Adobe Flash and Sun Java are both examples of plug-ins and are each downloaded from their own websites. Often a website that requires a plug-in has a link to the appropriate plug-in site that contains the download for the plug-in.