Good is an adjective. It describes nouns or pronouns. It may be used with descriptive linking verbs like look, feel, sound, taste, or be to describe the subject.
Incorrect: The coffee tasted well this morning.
Correct: The coffee tasted good this morning.
Correct: The pitcher is looking good today.
Well is normally an adverb. It describes verbs (sometimes adjectives) and is used with most other verbs.
Well as an adjective means "healthy."
Incorrect: He pitches good.
Correct: He pitches well.
Incorrect when speaking specifically of health: I do not feel very good.
Correct: I do not feel very well. (healthy)
See also Irregular Comparisons.