According to Curiosity.com, Isaac Newton's work focused on several topics which eventually became the basis for the entire field of classical physics. While he had many achievements, his most important theories included those that governed the laws of motion and the foundations of calculus.
Newton was the first person to adequately form a theory that encompassed the movement of bodies through space, including the principles of inertia, action, reaction and acceleration. Furthermore, he displayed through theoretical mathematics that the motion of objects could be effectively predicted. He was also the first to describe the force of gravity, noticing that all things attracted all other things in space and that the planets orbited the Sun by way of an invisible force, even though that force was not properly identified until Einstein developed his theory of relativity. In his most important publication, "Principia," Newton basically explained for the first time why things moved the way they do.
Newton also made a large contribution to the engineering of telescopic lenses. Prior to Newton, the glass lenses that made up most telescopes of the day produced fuzzy images at the edges of bright objects. Newton discovered that utilizing mirrors in his telescope would eliminate the fuzziness, and his telescope could outperform those with lenses that were far bigger.