Whats Difference between "Add-ins" "Add-ons" "Plug-ins"

Hi friend as per my knowledge there is no difference between these words i mean plug ins ,plug-on ,add-in and add-on same.just some are using differently.i think add on more appropriate and plug in is the perfect word.

it is quite difficult to say which one is the correct and most meaningful as all above words are used in different contexts . As you are asking this about computer software : for firefox either a add-on or an extension is used . plug-in is used for the winamp music player , Adobe products , VLC media player , photo and video editing software ... etcetra . plug-in is generally used in hardware (electrical , electronics & mechanical) so far not heard of a plug-on . add-in is also not a popular word .