Differential K-theory is a refinement of topological K-theory generated by vector bundles with connection. This is a largely unexplored invariant of a manifold, which has gained interest within mathematical physics since Freed and Hopkins proposed that it is the correct home for Ramond-Ramond fields. It has gained a wider audience as progress is made on an index theory and on an axiomatic characterization as a "differential cohomology theory". In this talk I will give an overview of the subject, and then describe joint work with Ralf Meyer in which we view differential K-theory as a Picard groupoid valued 2-functor.