For a long time scientists classified living things into five kingdoms: animals, plants, fungi, protists and prokaryotes. The groups were based mainly on features that can be seen, such as cell structure and body shape.
Later, scientists could compare the genetic material of organisms, for example the sequence of bases in the genes for their ribosomal RNA. Carl Woese and his colleagues used genetic analysis and found that the prokaryotes were not one group. Some of them are very different from bacteria, and these are now called archaea.
This led to the three domain system: Archaea, Bacteria and Eukaryota. Archaea are single-celled prokaryotes, and many live in extreme places such as hot springs. Bacteria are the other prokaryotes. Eukaryota includes animals, plants, fungi and protists, which all have cells with a nucleus.