In the Meditations, Descartes describes mind as a thinking thing, and the body something with extension.
To Descartes, ideas are like images of the real world. They reside in the mind, and do not necessarily perfectly replicate actual objects.
To prepare for his proof of God, Descartes distinguishes between formal reality and objective reality of ideas. Specifically, objective realities of ideas differ based on the level of perfection of the object they represent. Nothing has more objective reality than the idea of God.