Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a promising technology for infrared photon counting, offering high efficiency, low dark counts, and excellent timing resolution. This review covers the basic operating principles, device behavior models, and the evolution of SNSPD design. It discusses improvements in performance, including coupling efficiency, absorption efficiency, and registering efficiency. The review also addresses practical considerations such as cooling, optical coupling, and readout circuits. Additionally, it explores advanced applications of SNSPDs, including quantum key distribution (QKD), optical quantum computing, characterization of quantum emitters, classical space-to-ground communications, integrated circuit testing, fiber temperature sensing, and time-of-flight depth ranging. The goal is to provide a comprehensive snapshot of an emerging superconducting detector technology on the threshold of maturity.Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a promising technology for infrared photon counting, offering high efficiency, low dark counts, and excellent timing resolution. This review covers the basic operating principles, device behavior models, and the evolution of SNSPD design. It discusses improvements in performance, including coupling efficiency, absorption efficiency, and registering efficiency. The review also addresses practical considerations such as cooling, optical coupling, and readout circuits. Additionally, it explores advanced applications of SNSPDs, including quantum key distribution (QKD), optical quantum computing, characterization of quantum emitters, classical space-to-ground communications, integrated circuit testing, fiber temperature sensing, and time-of-flight depth ranging. The goal is to provide a comprehensive snapshot of an emerging superconducting detector technology on the threshold of maturity.