Kepler's Laws (i.e., Kepler's Laws Of Planetary Motion) are three laws that cover the basic Kinematics (description of rules governing the motions) of the Solar System's planets, and are useful for many cases of orbiting bodies. Johannes Kepler worked them out in the 16th century, in his effort to find a mathematical basis for the known astronomy.
As quoted here, the laws would apply (as a special case) to a circular orbit with constant orbital speed.
The laws are generally useful for bodies orbiting around others, yielding very good approximations given some assumptions, among them that the "central" body is by far the most massive, that the bodies are basically spherical, that Gravitational interactions between bodies orbiting the central body are minimal, and that Relativistic Speeds are not involved. Orbits that do not match these laws yield information when analyzed to uncover which assumptions don't hold and to what degree.