All biological samples emit ultra-low intensity light without any external stimulation. Recently, scientific communities have paid particular attention to this phenomenon, known as ultra-weak photon emission (UPE). UPE has been introduced in the literature as an alternative for biophoton, low-level chemiluminescence and ultra-weak bioluminescence, while it differs from ordinary bioluminescence, fluorescence and phosphorescence. Some UPE parameters including spectrum and intensity have been already recognized, while other features such as the main origin(s), statistical distribution and fractality of UPE are partially understood. Ultra-weak photon detection has a broad range of potential applications in different industries such as agriculture and medicine. The correlation between UPE and physiological state of a system facilitates the use of UPE as a completely non-invasive diagnostic method in cases such as cancer detection. In this review article, we aimed to provide useful information on specific characteristics, possible origin(s) and potential applications of UPE. Moreover, we introduced some physical models for UPE and presented several controversial hypotheses in this context.