What are plug-ins (software)?
They are programs that can easily be installed and used as a part of your web browser.
What are add-ons or extensions (software)?
Browser extensions extend your web browser with additional features, modify web pages, and integrate your browser with the other services you use.
What languages can you use to write plug-ins?
Max
SyncEdit
Sonic Birth
Reaktor
What is the difference between plug-ins and add-ons?
An Add-on is a Theme or an Extension.
Plugins add new functionality to an application, such as viewing special graphical formats or playing multimedia content in a web browser.
What are widgets?
Widget is a generic term for the part of a GUI that allows the user to interface with the application and operating system.