Photonic nanojet (PNJ) is a high intense, tightly focused light beam on the shadow side surface of lossless dielectric microparticles when the size of the particle is approximately equal to or slightly larger than the wavelength of the incident light. PNJ exhibits exceptional characteristics, including higher intensity than the incident light, the minimum full width at half-maximum less than the diffraction limit of the beamwidth, propagation beyond the evanescent field region and strong backscattering. These properties make PNJ crucial in various applications such as optical signals enhancement, micro-/nano- processing and manufacturing, super-resolution optical imaging, ultra-sensitive trapping and sensing, among others. This review article begins by introducing the origins and discovery of PNJ. Subsequently, it provides an elucidation of the model, theory, morphology features, experimental measurements, and key properties of PNJ. Furthermore, the study investigates and discusses several crucial applications of PNJ. Finally, a comprehensive summary and outlook for PNJ are presented.