What is the difference between "person" and "substance"?

According to the answer here : >St. Thomas Aquinas explained the preceding definition [of person] in terms that practically constitute a new definition: **a substance**, complete, subsisting per se, **existing apart from others** (Summa Theologica, III, Q. xvi, a. 12, ad 2um). According to trinitarian theology, the triune god is said to be "three seperate persons" consisting of "one substance". Does this mean the triune god is "three separate substances, each existing apart from other substances, consisting of one substance"? In other words, what are the key differences between a "person" and a "substance"?