Archaea are not bacteria, but prokaryotic microbes that occupy their own unique domain and are involved in virtually every chemical cycle in the environment. They can be found in some of the most extreme environments on earth and play vital roles in many of the geochemical cycles on which other organisms depend. Archaea are able to extract nitrogen from ammonia, capture carbon and both generate and oxidize methane.
The academic importance of Archaea cannot be overstated. The discovery in the 1970s of this domain of life greatly expanded biologists' understanding of the origin of life on earth. One theory is that life began in high-temperature environments, and that the ability to live below the boiling point of water developed well after the first microbes evolved. This particular theory was inspired by extremophile archaeans, the archaeans that thrive in extreme conditions typically detrimental to life.
Archaeans are also important to taxonomists. The basic unit of taxonomy is the species. Among eukaryotic life forms, a species is a group with gene flow between its members, but which doesn't permit gene flow outside of the group. Archaeans, however, frequently exchange genes across what would normally be species boundaries. As a result, some taxonomists and geneticists are reconsidering the common definition of species across the three domains of life.