ADJECTIVE: A word used to modify a noun or pronoun.
(To MODIFY a word means to describe the word, or to make its meaning more definite.)
An ADJECTIVE MODIFIES a word by telling WHAT KIND, WHICH ONE, HOW MUCH, or HOW MANY.
ADVERB: A word used to modify a verb, an adjective or another adverb.
An ADVERB MODIFIES a word by telling WHERE, WHEN, HOW, or TO WHAT EXTENT (HOW MUCH or HOW LONG).