'Good' is an adjective. 'Well' is the adverb form of 'good'.

Linking verbs: A linking verb connects a subject with a verb. It does not express action. Some examples are the verb “to be”: (present) am, is, are (past) were Action verbs: An action verb expresses action – for example: I run, I cook, I watch ---------------------------------------------------------------------------------------------- When you are dealing with “good” (adjective) and “well” (adverb) – there are a couple of important rules to remember: You can use an adjective after a linking verb, but not after an action verb. When used in this way , the adjective is modifying the subject (I, He, We, etc). I am tall. He is nice. We are happy. I am good. You can use an adverb after a linking verb. You can also use an adverb after an action verb. When used in this way the adverb is modifying the verb. As an adverb “well” means that the action is done in a good manner – or with skill. I run quickly. (quickly modifies run) I cook well. (well modifies cook) I am doing well. (well modifies doing) – This means the same thing as “I am good”. The word "well" is special because it can be used as both an adverb and an adjective. When used as an adjective, well modifies the subject (I, He, We, etc). As an adjective “well” usually means that someone or something is without illness. I am well. (I am not sick)

Thanks Stacey! :).

In my opinion, well and good have a slightly different meaning in conversation. Well implies physical well being. You generally hear, "I am doing well" with regards to health. However you hear "good" with readers to the person's general feeling in that moment.