Autonomic computing is an emerging research hotspot, which aims at hiding system management complexity from human users by means of "technologies managing technologies", to establish guidable, state-aware, and self-adaptive computer systems. Currently, the research of autonomic computing is still in its infancy, without systematic and mature theories. After clarifying the definition of autonomic computing, this paper proposes a conceptual model, which describes the basic working mechanisms and principles of autonomic elements and autonomic computing systems. On the basis of this model, two categories of autonomic computing systems based on knowledge model and mathematical model respectively are summarized and analyzed, with their advantages and disadvantages. Finally, the future research directions of autonomic computing are proposed.