Scientific classification is the system used by biologists to classify all life on Earth. It is also known as scientific classification in biology or Linnean classification, after Carolus Linnaeus (1707-1778), who first attempted this huge project. The pioneering work was Systema Naturae, first published in 1735, which ran through twelve editions throughout Linnaeus' lifetime. The first version had a few thousand entries, grouped into taxa based on shared physical characteristics. Today, over two million species are recognized by science, though the total number of plant, animal, and unicellular species on Earth is estimated at between 10 and 100 million.
The system of scientific classification used in biology is hierarchical, with eight levels of categorization. Moving from smallest to largest, they are: species, genus, family, order, class, phylum, kingdom, and domain. Even more divisions are often added in between these levels, as life itself has no special obligation to be categorized naturally in just eight hierarchical levels. To give an example of each, consider the position of humans in scientific classification. Humans are species Homo sapiens, genus Homo, family Hominidae (the great apes), order Primates, class Mammalia (mammals), phylum Chordata (vertebrates and a few close relatives), kingdom Animalia (animals), domain Eukarya (eukaryotes, organisms with complex cells).
The highest levels of scientific classification have changed several times over the years. In 1735, Linnaeus introduced two kingdoms: Vegetabilia (plants and fungi) and Animalia. In 1866, the German biologist Ernst Haeckel introduced another kingdom, Protista, for all unicellular organisms. In 1937, French biologist Edouard Chatton divided in life into two "empires" -- Prokaryota and Eukaryota, based on more detailed observations of cells in plants, animals, and bacteria. It turned out that plants and animals had fundamental similarities in the complexity of their cells and the presence of cellular nuclei, whereas bacteria lack both nuclei and organelles (intracellular structures).
Several more major updates followed in the 50s, 60s, and 70s, which were a time of fast progress in biology. Herbert Copeland updated the system to four kingdoms in 1956, when he renamed Prokaryota to Monera and split Eukaryota into three kingdoms: Protista, Plantae, and Animalia. Another change came in 1969, when Robert Whittaker split Protista into Fungi and Protista, giving fungi their own top-level classification for the first time.
In 1977, Carl Woese and his collaborators introduced the most crowded top-level system yet, with six kingdoms: Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and Animalia. Then, in 1990, the system was mercifully simplified by Woese, decreased to three domains: Bacteria, Archaea, and Eukarya. The latter is the current top-level classification system used.