Despite the relevance to most aspects of crystallization, a comprehensive understanding of the kinetics of heterogeneous nucleation has not been well established yet. In this paper, a new kinetic model based on the "steady-state" approach will be put forward to describe both heterogeneous and homogeneous nucleation. As a key point in this model, the effect of foreign particles on both the nucleation barrier, the "chain reaction" process and the transport of structural units is taken into account. Ranging from low to high supersaturations, heterogeneous nucleation plays a more comprehensive role in nucleation than we expected, depending on the size of foreign particles and the interaction and structural matching between foreign particles and the nucleating phase. It follows that genuine homogeneous nucleation may only be possible at very high supersaturations, and can become easier when convection is eliminated. The results have been verified by the CaCO3 nucleation experiments. This general model allows us to describe many unexpected nucleation phenomena for the first time, and will have significant impact on the control of nucleation, understanding of crystallization in microgravity, the measurement of the crystal-fluid interfacial tension, and crystallization in general.