An adverb is a word that modifies verbs, adjectives, other adverbs, phrases, or clauses by indicating manner, time, place, cause, or degree. Adverbs answer questions like how, when, where, or how much. Examples show adverbs modifying verbs like "quickly" and "patiently", and entire sentences like "unfortunately". Adverbs are also distinguished from adjectives in examples where words like "late", "fast", and "hard" function differently based on context.