Over the time life has evolved three main lines, known as the three major domains: bacteria, archaea and eukaryotes. This is a hierarchical division in which similar organisms are divided into suitable groups based on their characteristics. The smallest hierarchical unit is the species, followed by genus, family, order, class, phylum, kingdom and domain.
Explanation for children:
A domain is a very large group of organisms, which is then divided into smaller and smaller groups. You can imagine, for example, that games are a domain. The games can be divided into computer games, board games and so on. The computer games differentiates in single-player games and multi-players games. This division goes on and on until you have arrived at the smallest possible unit, which is the species in biology. The largest unit, the domain, is divided in the following three: eukaryotes, bacteria and archaea.