Johannes Kepler lived from 1571 to 1630 and is famous for his contribution to astrophysics. For an explanation of Kepler's 3 Laws of Planetary Motion watch this video or the one below.
The Law of Ellipses
Kepler's first law - sometimes referred to as the law of ellipses - explains that planets are orbiting the sun in a path described as an ellipse. The path of the planets about the sun is elliptical in shape, with the center of the sun being located at one focus.
The Law of Equal Areas
Kepler's second law - sometimes referred to as the law of equal areas - describes the speed at which any given planet will move while orbiting the sun. The speed at which any planet moves through space is constantly changing. A planet moves fastest when it is closest to the sun and slowest when it is furthest from the sun. An imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal intervals of time.
The Law of Harmonies
Kepler's third law - sometimes referred to as the law of harmonies - compares the orbital period and radius of orbit of a planet to those of other planets. Unlike Kepler's first and second laws that describe the motion characteristics of a single planet, the third law makes a comparison between the motion characteristics of different planets. The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun.
Related reading: Johannes Kepler; Man of Science, Man of God