How did St. Thomas Aquinas explain the difference between essence and existence?

St. Thomas Aquinas explained the difference between essence and existence by positing that they are distinct principles in all created beings, whereas they are identical in God.12

For Aquinas, existence is the act of being (esse). He taught that God is "pure being," meaning that in God, essence and existence are one and the same. In contrast, all other creatures possess a "real composition" of essence and existence. This means that a creature’s essence—what it is—does not inherently include the act of existing; rather, creatures participate in existence according to the limitations of their specific essence.1

By maintaining this distinction, Aquinas provided a fundamental metaphysical framework to differentiate the Creator from the created: only God possesses existence as his very nature, while for all other beings, existence is a perfection received and held in accordance with their essence.12