truck vs fire truck

truck and fire truck both are nouns.

Word	Noun	Adjective	Verb	Adverb
truck	Yes	No	Yes	No
fire truck	Yes	No	No	No

As nouns, fire truck is a hyponym of truck; that is, fire truck is a word with a more specific, narrower meaning than truck:

truck: an automotive vehicle suitable for hauling
fire truck: any of various large trucks that carry firemen and equipment to the site of a fire

truck (noun)	fire truck (noun)
an automotive vehicle suitable for hauling	any of various large trucks that carry firemen and equipment to the site of a fire
a handcart that has a frame with two low wheels and a ledge at the bottom and handles at the top; used to move crates or other heavy objects

truck (verb)	fire truck (verb)
convey (goods etc.) by truck