Optical frequency comb (OFC) has coherently bridged the gap between light and microwave. Its advent has brought revolutionary progress to the accurate measurements of optical frequency and time, and profoundly promoted the technological development of technology of the contemporary world. The earliest optical frequency combs are generated from mode-locked laser systems. However, optical frequency combs based on mode-locked lasers have typically been limited to laboratory applications, due to their complexity, large size, and high cost. In recent years, a new type of optical frequency comb has emerged to address these problems. It is excited by continuous-wave laser coupling into a high-quality optical microresonator, generating equidistant sidebands in the frequency domain through four-wave mixing, and achieving mode locking in the time domain by using nonlinear effects to balance dispersion. This novel optical frequency comb is named "microcombs". Compared with traditional optical frequency combs, microcombs offer advantages such as compact size, integrability, low power consumption, and a wide repetition frequency range. Their occurrence marks the era of the generation of optical frequency combs towards chip-scale size and has aroused increasing attention from the scientific and industrial communities. This paper is ended by summarizing the current challenges faced by microcombs and giving a prospective outlook on their future development.