What is Plug-in ?

In computing, a plug-in (Plugin, extension, or add-on / addon) is a software component that adds a specific feature to an existing software application. When an application supports plug-ins, it enables customization. The common examples are the plug-ins used in web browsers to add new features such as search-engines, virus scanners, or the ability to utilize a new file type such as a new video format. Well-known browser plug-ins includes the Adobe Flash Player, the QuickTime Player, and the Java plug-in, which can launch a user-activated Java applet on a web page to its execution a local Java virtual machine.

Add-on (or addon) is the general term for what enhances an application. It comprises snap-in, plug-in, theme and skin. An extension add-on tailors the core features of an application by adding an optional module, whereas a plug-in add-on would tailor the outer layers of an application to personalize functionality.

A theme or skin add-on is a preset package containing additional or changed graphical appearance details, achieved by the use of a graphical user interface (GUI) that can be applied to specific software and websites to suit the purpose, topic, or tastes of different users to customize the look and feel of a piece of computer software or an operating system front-end GUI (and window managers).

It helps to enable third-party developers to create abilities which extend an application, support easily adding new features, reduce the size of an application and separate the source code from an application because of incompatible software licenses.