We investigate the cluster growth and structures, the magnetic transitions and the rheological characteristics of ferromagnetic colloidal systems. We develop a Brownian dynamics calculation method for the dynamic analyses, in which the translational and rotational motions of ferromagnetic particles are taken into account, and also a biased Monte Carlo method for the analysis of the equilibrium cluster and magnetic structures. Through the analyses, the following results have been obtained; (1) A dynamic scaling law applies to the cluster growth process and the fractal dimension of the clusters is constant during the growth process. (2) The fractal dimension of two-dimensional clusters is 4/3 irrespective of the value of the control parameter when it is not very large, whereas that of three-dimensional clusters changes from 5/3 to 2 with an increase in the control parameter. (3) When the number density of ferromagnetic particles is high, a magnetic transition, which is different from a second-order phase transition, occurs. The specific heat and the magnetic susceptibility become maximum at the transition point, but the values of them do not change with the system size. Instead, the nonlinear magnetic susceptibility shows negative divergence at the transition point. (4) When the colloidal system is subjected to both shear flow and external magnetic fields, the increase in the apparent viscosity is caused by the suppression of the rotational motions of the particles in the case of weak dipole-dipole interactions, whereas it is caused by the cluster formations in the case of strong dipole interactions.