The study of b quarks has now reached a stage where it is useful to review what has been learned so far and also to look at the implications of future studies. The most important observations thus far -- measurement of the "B" lifetime, B0 -- B0 mixing, and the observation of b→ u transitions, as well as more mundane results on hadronic and semileptonic transitions -- are described in detail by experimentalists who have been closely involved with the measurements. Theoretical progress in understanding b quark decays, including the mechanisms of hadronic and semileptonic decays, are described. Synthesizing the experimental and theoretical information, the authors discuss tests of the standard model and measurements of standard model parameters. Possibilities of more extensive tests using measurement of CP violation in the B system are also addressed.