Plug-in, Add- on, Widgets

What are plug-ins?
is a software component that adds a specific feature to a  computer program.

What are add-ons or extensions (software)?
an accesorios device or piece of software designed to increase the capability of a computer or hi-fy system

What languages can you use to write plug-ins?
HTML, PHP and CSS

What is the difference between plug-ins and add-ons?
plug ins are for the features and add ons customizing the app

What are widgets?
is a small application with limited functionality that can be an auxiliary app