Extremophiles, also known as archaea, are single-celled, prokaryotic organisms which thrive in extreme conditions which would otherwise be toxic to life These  environments include those with extremely high temperatures or salinities.  They are believed to be some of the oldest forms of life on Earth because their fossils can be found in ancient sedimentary rocks (hence their name).  They are important in astrobiology because extreme conditions, such as that of the early Earth, are far more common in the universe than majority of Earth's environments.  Extremophiles may very well be the most common form of life throughout the cosmos.