Monte Carlo simulations are used to get an insight into the formation of fractal aggregates from diluted to concentrated colloidal particle dispersions. Using irreversible conditions, we investigate the aggregation size distribution, architecture of the resulting fractal aggregates, possible transitions from simple aggregation to percolation and from percolation to the homogeneous aggregation regime, and discuss the fractal dimension determination from the radial distribution function. In particular the effects of the particle concentration on the aggregate fractal dimensions are considered. Reversibility is also introduced in the model so as to consider more realistic systems. The effects of aggregate fragmentation and internal reorganization are then investigated by adjusting the interparticle interaction potential. Important results dealing with the concomitant effect of aggregate break-up and internal reorganization on the aggregate local structure and stability with regards to phase separation are discussed.