Six kingdoms of life exist: archaebacteria, eubacteria, protists, fungi, plants and animals. In the past, the two bacterial kingdoms were placed into one kingdom, but the two types of bacteria were found to be so different that they warranted separate kingdoms.
When the first system of classification was devised by Carolus Linnaeus, organisms were placed into one of two kingdoms, plant or animal. As technology advanced and scientists increased their knowledge of different species, other kingdoms were added. For example, fungi were initially placed with plants but now belong to their own kingdom.
The kingdom with the most numerous species is the animal kingdom with one million species known. Viruses do not belong to any of the six kingdoms.