What is the main difference between Kepler's system and Copernicus system?

Copernicus's theory has the Sun at the centre but uses a model of circles and epicycles in the same way that the ancient Ptolemaic one used.

Kepler continued to use the idea of having the Sun at the centre but, with the help of Tycho Brahe's new accurate observations, Kepler discovered that elliptical orbits allowed a better fit to the observed positions.

Tycho instructed Kepler to work on the orbit of Mars, which was a real piece of luck because Mars's orbit is more elliptical than the other planets' orbits (except Mercury). This led Kepler to the idea of the ellipse, which is the model used today.