Spectroscopic characteristics of atoms and ions in plasma are conveniently understoodwhen we classify these atoms and ions into the ionizing plasma, recombining plasma andionization-balance plasma, according to their ionization ratios. For each class the population characteristics are reviewed together with the dominant populating and depopulating processesfor excited levels. For some of the low density conditions, the excited-level populations tend tobe very close to the thermal equilibrium values. This fact is contrary to the general expectation. Relationships between the atomic processes responsible to these populations are discussed. Forhigh-density ionizing plasma the population density distribution is established that is inverselyproportional to p-6 where p is the (effective) principal quantum number of the level. Theorigin of this distribution is explained for hydrogenic cases, but is not known for other atomsand ions having more complicated energy-level structures.