Before revision: A chemical reaction network is a chemical system involving multiple reactions and chemical species. The simpleststochastic models of such networks treat the system as a continuous time Markov chain with the state being the number of molecules of each species and with reactions modeled as possible transitions of the chain.
After revision: The simplest, and most common, stochastic model for population processes, including those from biochemistry and cell biology, are continuous time Markov chain with the state being the number of molecules of each species and with reactions modeled as possible transitions of the chain.