The extremely low quantum noise characteristics of masers play an important role as ultra-low noise preamplifiers and microwave clocks in fields such as wireless communications, deep space exploration, navigation, and time-frequency measurements. A detailed overview of the fundamental principles and developmental trends of masers is provided. Building upon the research progress of typical cryogenic ruby solid-state masers, it summarizes their advantages in low-noise microwave amplification, high-stability microwave oscillation, and ultra-sensitive nuclear magnetic resonance. To address the challenge of the traditional solid-state masers that can only operate in cryogenic environments, the novel maser formation mechanisms and gain media have been explored. It reviews recent domestic and international research efforts aimed at achieving the operation of solid-state masers at room temperature and discusses the research difficulties and future development trends of room-temperature solid-state maser.