What are the criteria for comparing unicellular organisms?

Single-celled eukaryotes are fully functional systems capable of self-replication given a suitable environment. This is not the case for most multicellular eukaryotes, although there are many exceptions.

The simplest multicellular eukaryotes are just clusters of a number of unicellular organisms (for example, Volvox). In an organism like a Volvox, all cells containing this creature (or, more precisely, a colony) are identical. All cells are essentially doing the same thing, they don’t have different roles.