IRS vs Internal Revenue Service

IRS and Internal Revenue Service both are nouns.

Word	Noun	Adjective	Verb	Adverb
IRS	Yes	No	No	No
Internal Revenue Service	Yes	No	No	No

As nouns, IRS and Internal Revenue Service are synonyms defined as:

IRS and Internal Revenue Service: the bureau of the Treasury Department responsible for tax collections

IRS (noun)	Internal Revenue Service (noun)
the bureau of the Treasury Department responsible for tax collections	the bureau of the Treasury Department responsible for tax collections