  We study Cheeger-Simons differential characters and provide geometric
descriptions of the ring structure and of the fiber integration map. The
uniqueness of differential cohomology (up to unique natural transformation) is
proved by deriving an explicit formula for any natural transformation between a
differential cohomology theory and the model given by differential characters.
Fiber integration for fibers with boundary is treated in the context of
relative differential characters. As applications we treat higher-dimensional
holonomy, parallel transport, and transgression.
