'Adjective' modifies a noun . For example , see the sentence "John purchased a nice bag" .In this 'nice' is the adjective which modifies the noun 'bag' .
'Adverb' modifies a verb , adjective or even the adverb itself . Consider this as an example "John swiftly passed the ball to Sam" . In this sentence 'swiftly' is the adverb which modifies the verb 'passed '