Systematics aims to define species and sort them into a hierarchy of named groups congruent with the branching pattern of evolution. There is no single operational definition of what a species is, and taxonomy at all levels is subject to change with new methods and data, but species diversity of better known organisms can often be assessed with useful accuracy. Globally, about 1.75 million species have been described and named, but the total including undescribed species might be up to ten times greater.

All species known are assigned on the basis of shared patterns of form and function to one of about 100 major groups (phyla). There are marked differences between phyla in overall morphology, physiology and mode of life. These differences imply the existence of major genetic diversity, and contribute directly to structural, trophic, and other dimensions of diversity within ecosystems.

The phyla of living organisms fall into three primary lineages: the true bacteria, the archaeans, and other organisms. The first two are prokaryotes, the remainder (protoctists, animals, fungi and plants) are eukaryotes.