what is the difference between modal and auxiliary verbs?

An auxiliary verb adds meaning to the clause in which it comes. The meaning may be grammatical or functional. It usually comes with the main verb. In very simple terms, an auxiliary verb is a helping verb.
For example: He has eaten his food. He is sleeping now.

Modal verbs are a type of auxiliary verbs. they express modality such as possibility or obligation. Some modals are 'can, could, ought, will'.
By definition, all modals are auxiliaries. But, only some auxiliaries are modals.