Graphene with large grain size and high electronic mobility was synthesized by ambient-pressure chemical vapor deposition on platinum and transferred to a variety of substrates for characterization by electrical transport, Raman spectroscopy, and transmission electron microscopy. The grain boundaries and pyramid-like multilayer structures of graphene samples prepared in this way were imaged with dark-field transmission electron microscopy, and a method was developed to use differences in first- and second-order diffraction intensities to characterize the layer-number and stacking-order of graphene up to at least seven layers. Combining this dark-field method with secondary electron microscopy, electron backscatter diffraction, Raman microscopy, and electronic transport measurements, it was also discovered that nano-crystalline carbon impurities distributed inhomogeneously under mono-layer graphene. These impurities were distributed inhomogenously, exhibiting micron-sized islands of denser impurity concentration whose shapes depended on the orientation of the grains of the Pt substrate. In such impurity-decorated samples both linear and quadratic magnetoresistance was observed. The linear magnetoresistance was found for carrier densities well beyond filling the ground Landau level, therefore Abrikosov's quantum magnetoreistance is ruled out. Sample 1, 2, and 3 with suppressing inhomogeneity were synthesized by controlling growing conditions in the chemical vapor deposition process. The magnetoresistance positively correlates with the density of inhomogeneity. The magnetoresistance in samples with widely varying impurity concentrations can be described by a unique function of the ratio of carrier-density inhomogeneity to gate-induced carrier density, and can therefore be attributed to impurity-induced inhomogeneity.