K-theory is a valuable tool with applications in various areas of mathematics, including topology, geometry, algebraic geometry, and number theory. In this talk I will attempt to give a rough outline of what K-theory is about, in the algebraic as well as the geometric context. Given a ring R and a space X, I will define the groups K(R) and K(X), and explain the relation between the two. I also hope to outline some applications in various fields. This talk should be accessible and I hope interesting to students of both algebra and geometry alike.