The mechanisms of recombination of photo-generated charge carriers and the origin of luminescence in silicon nanocrystals, quantum dots or nanoparticles (Si-NPs) are essential for the application of these materials in future nano-optoelectronics. In the last two decades, these issues have been lively debated in the literature, with different investigations reporting seemingly contradicting results. With the study presented here, we clarify the dominant electron-hole recombination mechanisms and the origin of light emission in Si-NPs with different forms of surface passivation, namely H-terminated and (photo-)oxidized. We find that, independently of surface passivation, at low excitation powers recombination of photo-carriers is dominated by monomolecular channels via defect states, while at relatively high excitation electron-hole bimolecular recombination dominates. Moreover, we establish that in H-terminated Si-NPs, light emission is mainly due to recombination involving sub-band gap electronic states at the Si core of the NPs, whereas in surface (photo-)oxidized Si-NPs a two-component luminescence is observed, originating from Si core and oxide-related states, respectively. Further, we discuss the critical sensitivity of H-terminated Si-NPs to photo-oxidation even under low oxygen and water molecule environment and its consequences for Si-NP applications. Photo-oxidation is described within the Cabrera-Mott chemical reactions theory.