In computing, a plug-in is a software component that adds a specific feature to an existing computer program.
When a program supports plug-ins, it enables customization.
The common examples are the plug-ins used in web browsers to add new features such as being able to download videos from youtube, virus scanners, or the ability to use a new file type such as a new video format.
Well-known browser plug-ins include 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 on a local Java virtual machine.
A theme or skin 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).