An adjective is a word that describes or modifies a noun or pronoun, such as "happy," "big," or "cold." Adjectives typically answer the questions "What kind?" "Which one?" or "How many?"
An adverb, on the other hand, is a word that describes or modifies a verb, adjective, or another adverb. Adverbs usually answer the questions "How?" "When?" "Where?" "To what extent?" or "Why?" For example, in the sentence "She ran quickly," the word "quickly" is an adverb modifying the verb "ran."
In summary, adjectives describe nouns, while adverbs describe verbs, adjectives, or other adverbs.