Introduction to what is an Add-in, Add-on or Plug-in and their usages.
What is an Add-in: These are additional programs that a user can install separately to enrich the capabilities of a larger software program which is already installed.
Why is it important: The importance of an add-in can be explained as simple as people opting for a power steering or a power window when they are buying a car. A car can be driven even without a power steering or a power window but these additional features will enrich the performance of the car or make your driving more comfortable. Add-ins are something that provides additional features.
An add-in can not work by its own it goes along with a larger software program. There is no point in thinking about a power steering when a car itself is not there. 😛
Add-ins are sometimes referred as add-on or plug-in.
Adobe Flash Player and Acrobat is a commonly used plug-ins in web browsers.