If you’re healthy, are you well or good? If you’re happy, are you well or good? If you score a lot of touchdowns in football without getting a black eye or a broken leg, are you playing good or well?
Good is an adjective. Adjectives describe—tell something about—nouns or pronouns; that is, they tell what kind of, as in a green football jersey, an over-rated quarterback, or a selfish decision.
I’m in good spirits means I’m happy (what kinds of spirits am I in today?). I’m good at football tells what kind of player I am. I have a good grasp of the concept tells what kind of grasp I have.
Well is an adverb when it describes how something is being done. He doesn’t play football as well as his reputation suggests tells how he plays the game (and means that the media distorts his ability). I play football well tells how I play the game.
Well is an adjective, however, when it refers to the state of health. I don’t feel well today. To say I feel well today means I feel healthy today; don’t confuse it with I feel good today, which means (see above) I’m in good spirits today.
Sherry
Paul’s book–Literally, the Best Language Book Ever;
Everything Language and Grammar 