Perhaps they are best defined as eukaryotes that are NOT fungi, animals, or plants. Fungi Fungi are a eukaryotic, heterotrophic, usually multicellular group having multinucleated cells enclosed in cells with cell walls. They obtain their energy by decomposing dead and dying organisms and absorbing their nutrients from those organisms. Plantae Plants are immobile, multicellular eukaryotes that produce their food by photosynthesis and have cells encased in cellulose cell walls. Animalia Animals are multicellular, heterotrophic eukaryotes that are capable of mobility at some stage during their lives, and that have cells lacking cell walls.
Animals provide food, clothing, fats, scents, companionship, and labor.
An exhaustive list of species. A long-loading page, but well worth the wait. A collection of websites focusing on the diversity of life. Taxonomy of Life A listing of taxa, but not much beyond that. The nomenclature rules for plants, algae, and fungi.