A eukaryote is an organism with a complex cell or cells, in which the genetic material is organized into a membrane-bound nucleus or nuclei. Eukaryotes (also spelled eucaryotes) comprise animals, plants, and fungiâwhich are mostly multicellular - as well as various other groups that are collectively classified as protists (many of which are unicellular). In contrast, prokaryotes are organisms, such as bacteria and archaea, that lack nuclei and other complex cell structures.