There are nouns, verbs, adjectives and adverbs (and more).
The teacher likes the book very much. (In the sentence, TEACHER is a noun, LIKE is a verb, BOOK is a noun again and VERY MUCH is an adverb. VERY is an adverb modifying another adverb MUCH and MUCH is an adverb modifying the verb LIKE.)
He walks slowly. (In this sentence, WALK is a verb and SLOWLY is an adverb.
She is very beautiful. (VERY is an adverb modifying adjective "beautiful)
Hopefully, he will come. (In this sentence, HOPEFULLY is an adverb modifying the whole sentence.)
So an adverb is a word used to modify a verb or another adverb or an adjective or sometimes a sentence.