Autophagy is an intracellular degradation system that delivers cytoplasmic constituents to the lysosome. Despite its simplicity, recent progress has demonstrated that autophagy plays a wide variety of physiological and pathophysiological roles, which are sometimes complex. Autophagy consists of several sequential steps—sequestration, transport to lysosomes, degradation, and utilization of degradation products—and each step may exert different function. In this review, the process of autophagy is summarized, and the role of autophagy is discussed in a process-based manner.