Plug-Ins and Add-Ins
Plug-Ins and Add-ins are pieces of software, usually created by a third party software designer, that are designed to work as a supplement to a major application.
A perfect example of this would be the extra programs that your Internet Browser needs to browse certain websites. For instance, when you visit a website that has Flash content, your browser, the very first time you visit the site, will prompt you to install a small add-in in order to view the content.
Adobe Reader installs a web browsing add-in that allows you to view PDF files directly inside the frame of your internet browser.