wood louse vs pill bug

wood louse and pill bug both are nouns.

Word	Noun	Adjective	Verb	Adverb
wood louse	Yes	No	No	No
pill bug	Yes	No	No	No

As nouns, pill bug is a hyponym of wood louse; that is, pill bug is a word with a more specific, narrower meaning than wood louse:

wood louse: any of various small terrestrial isopods having a flat elliptical segmented body; found in damp habitats
pill bug: small terrestrial isopod with a convex segmented body that can roll up into a ball

Other hyponyms of wood louse include sow bug.

wood louse (noun)	pill bug (noun)
any of various small terrestrial isopods having a flat elliptical segmented body; found in damp habitats	small terrestrial isopod with a convex segmented body that can roll up into a ball