The interest in plasma-activated water (PAW) has been greatly increasing during the last few years due to its potential applications in biological and medical sciences, as well as in agriculture and food industry. The effect of PAW is caused by the reactive oxygen and nitrogen species (RONS) generated in the liquid exposed to plasma. This work reviews recent research on the formation of reactive nitrogen species (RNS) in aqueous solutions treated by non-thermal plasma. The most important chemical reactions leading to the production of these active species in the gas and liquid phase, as well as their chemistry in the liquid, correlated with their lifetime are addressed. The literature data on the most important long-lived RNS in PAW (nitrite and nitrate) and the short-lived ones (peroxynitrite and peroxynitrate) are reviewed. The reported results show that their concentrations strongly depend on the type of electrical discharge, gas composition, liquid properties and treatment conditions, and thus tuning the PAW composition over a relatively wide range can be achieved. Results on the possible application of PAW in agriculture are also reviewed. The role of RNS in this area is related to their participation in various signalling pathways in plants, which regulate metabolic processes, plant development, response to stress, etc, and thus can finally lead to enhanced germination and/or faster germination process and increase in plant growth.