An adverb is a part of speech. It is any word that modifies any other part of language: verbs, adjectives (including numbers), clauses, sentences and other adverbs, except for nouns; modifiers of nouns are primarily determiners and adjectives.
Adverbs typically answer such questions as how?, when?, where?, To what extent?, In what kind or how often? 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. Adverbs also describe adjectives, verbs, and other adverbs.