A word that modifies a verb, an adjective or another adverb is called an adverb. For example:
She writes fast. ('fast' is an adverb that modifies the verb 'writes')
This is a very tall man. ('very'is an adverb that modifies the adjective 'tall')
He can speak quite loudly. ('quite' is an adverb that modifies that adverb 'loudly')