For example, when molecules of NaCl dissolve in water we don't really have any NaCl as such in the solution. We have Na and Cl ions because NaCl dissociates. We say the species present in solution are Na(aq) and Cl(aq). In this case, the species present in solution are the ions. In other cases, where the molecules don't dissolve in water, we would have molecular species in solution rather than ionic species.