An adverb is a part of speech. It is any word that modifies any part of speech or other verbs other than a noun (modifiers of nouns are primarily adjectives and determiners). Adverbs can modify verbs, adjectives (including numbers), clauses, sentences and other adverbs.
Adverbs typically answer questions such as how?, in what way?, when?, where?, and to what extent?. This function is called the adverbial function, and is realized not just by single words (i.e., adverbs) but by adverbial phrases and adverbial clauses. http://en.wikipedia.org/wiki/Adverb