Single-photon detectors (SPDs) are critical components in quantum communication, imaging, and high-resolution sensing. These detectors operate by converting photon interactions into measurable electrical signals via the photoelectric effect. Various types of SPDs offer unique advantages in sensitivity, speed, and efficiency, enabling applications such as quantum key distribution (QKD), single-photon imaging, and laser ranging. Recent advancements in SPDs have significantly improved quantum communication systems. Challenges like low-temperature operation and scalability limitations are being addressed via innovations in materials, structural designs, and cooling technologies. Emerging THz SPDs show promise in terahertz microscopy, quantum radar, and long-distance communication. SPDs are poised to revolutionize areas ranging from quantum communication to medical imaging.