English Grammar – Adverbs
The word adverb means joined to a verb. The adverb is the only word that can join to a verb to modify it.
When action is expressed, an adverb is usually added to define the action in some way, – time, place, or manner: as, “He began already to be proud of being a Rugby boy [time];” “One of the young heroes scrambled up behind [place];” “He was absolute, but wisely and bravely ruling [manner].”
But this does not mean that adverbs modify verbs only: many of them express degree, and limit adjectives or adverbs; as, “William’s private life was severely pure;” “Principles of English law are put down a little confusedly.”
An adverb, then, is a modifying word, which may qualify an action word or a statement, and may add to the meaning of an adjective or adverb, or a word group used as such.
Further reading: http://grammar.ccc.commnet.edu/grammar/adverbs.htm