In computing, a plug-in (or plugin) is a set of software components that adds specific capabilities to a larger software application. If supported, plug-ins enable customizing the functionality of an application. For example, plug-ins are commonly used in web browsers to play video, scan for viruses, and display new file types. Well-known plug-ins examples include Adobe Flash Player and QuickTime.

Add-on is often considered the general term comprising snap-ins, plug-ins, extensions, and themes.

Reference: Wikipedia.org