plug-in vs add-on

plug-in and add-on both are nouns.

Word	Noun	Adjective	Verb	Adverb
plug-in	Yes	No	No	No
add-on	Yes	No	No	No

As nouns, plug-in and add-on are synonyms defined as:

plug-in and add-on: a software module that extends or enhances the capabilities of an existing application

plug-in (noun)	add-on (noun)
a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities	a component that is added to something to improve it
a software module that extends or enhances the capabilities of an existing application	a supplementary component that improves capability
	a software module that extends or enhances the capabilities of an existing application