A large sample of D mesons, produced by the decay of the Ψ(3770) and observed by the Mark III detector at SPEAR, forms the basis for a study of the decays of charmed D+ and D0 mesons. Many Cabibbo-allowed and Cabibbo-suppressed decays are observed. When normalized by a new, absolute technique, the branching ratios appear significantly higher than those reported by previous experiments. No evidence is found for specific final states from D0 decay which are indicative of non-spectator W-exchange diagrams: limits are quoted. Finally, the inclusive semileptonic branching fractions of charged and neutral D mesons are measured by observation of electrons in the recoil from fully reconstructed hadronic D decays of known charm. By neglecting the contribution of Cabibbo-suppressed decays to the total decay widths, the ratio of these branching fractions can be interpreted as the ratio of D+ and D0 lifetimes, thus confirming the inequality of lifetimes observed by direct decay length experiments. The observed pattern of hadronic decays appears to favor modification of the spectator model over non-spectator processes as the main source of the lifetime difference.
