CPU board vs plug-in

CPU board and plug-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
CPU board	Yes	No	No	No
plug-in	Yes	No	No	No

As nouns, plug-in is a hypernym of CPU board; that is, plug-in is a word with a broader meaning than CPU board:

CPU board: the main circuit board for a computer
plug-in: a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities

Other hypernyms of CPU board include add-in, board, card, circuit board, circuit card.

CPU board (noun)	plug-in (noun)
the main circuit board for a computer	a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities
	a software module that extends or enhances the capabilities of an existing application