We measure with dynamic and static light scattering the radius of gyration, the hydrodynamic radius at various angles, and the average structure factor and determine the kinetic behavior for aggregation processes of a polystyrene colloid at moderately concentrated solid volume fractions (Phi(0) less than or equal to 0. 1 and various salt concentrations. In addition, we measure the gelation kinetics in these colloidal dispersions at volume factions Phi(0) greater than or equal to 0.05 using on-line oscillatory shear measurements. By modifying the surface of the sodium dodecyl sulfate stabilized polystyrene particles with Triton X-100, we show that the changed interaction potential is responsible for a clear change in the aggregation behavior and kinetics as well as in the gelation kinetics and gel properties. We demonstrate that the measured aggregation kinetics can be accurately described by population balance equations, i.e., the Smoluchowski aggregation equation, at all solid volume fractions investigated. The model for the aggregate structure allows us to distinguish between different fractal dimensions through comparison to the measured average structure factor. It is found that the structure of the aggregates is best described by a small fractal dimension of 1.8 and a power-law growth of the average radii. This is surprising for the relatively slow reaction-limited cluster aggregation regime with stability ratios of W similar to 1 x 10(6). A kinetic scaling in the aggregation behavior, independent of the salt concentration and the volume fraction, is found through a dimensionless time derived from the rate equations. Remarkably, the time evolution of the gel elastic modulus could also be scaled by the dimensionless time derived from the aggregation model. This finding suggests that the kinetics of gel formation still can be described as a second-order rate process, like aggregation, even for very concentrated systems close to the gel point.