In view of the inconclusive defect structure of CaCu3Ti4O12 (CCTO) ceramics, a defect model has been proposed in this investigation underlying the results of the previous works and updated knowledge in the literature papers concerning the band structure, charge transfer between Cu and Ti ions, and processing of CCTO. This model is capable of explaining why CCTO grain interior becomes semiconductive and grain and domain boundaries are insulating, and the grain-boundary resistance would dramatically drop due to the effect of sintering temperature/time and dopants. The ac conductivity of undoped and CaSiO3-doped CCTO associated with electrical charge carrier motion was investigated as a function of frequency at different temperatures. In order to identify the physical entity, the frequency dependence of ac conductivity at different temperatures is further characterized by the peak frequencies of the frequency dependence of the imaginary part of impedance (Z″) and electrical modulus (M″). The related physical entity has been specified by the activation energy and s value of power-law equation. It is suggested that each region cannot be specified by a unique physical entity; instead, it is a transition between relaxations of the related physical entity as the frequency increases. The electron hopping inside percolation clusters containing local polarized clusters of the related physical entity with the main contribution to the ac conductivity may determine the corresponding dielectric permittivity of each region.