PC board vs plug-in

PC board and plug-in both are nouns.

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

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

PC board: a removable circuit board for a personal computer; fits into a slot in the mother board
plug-in: a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities

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

PC board (noun)	plug-in (noun)
a removable circuit board for a personal computer; fits into a slot in the mother board	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