Photonic nanojets (PNJs) are high intense, tightly focused light beams on the shadow side surface of lossless dielectric microparticles when the size of the particles is approximately equal to or slightly larger than the wavelength of the incident light. PNJs exhibit exceptional characteristics, including higher intensity than the incident light, minimum beamwidth less than the diffraction limit, propagation beyond the evanescent field region and strong backscattering. These properties make PNJs 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 PNJs, followed by a description of their modeling, nanojet theory, morphology features, experimental measurements, and key properties. Subsequently, it conducts a survey and discussion of several important applications of PNJs. Finally, the article concludes with a summary and prospects for the future of PNJs.