An extremophile is an organism (a living thing) which lives best in extreme conditions that are harmful to most life on Earth. They are different from organisms that live in normal places, called mesophiles or neutrophiles.

In the 1980s and 1990s, biologists found that microbes can survive in extreme environments. These are niches that are extreme in some way. They may be extremely hot, or cold, or dry, or under huge pressure, or very salty or acidic. Complex organisms, such as animals or plants, cannot live in these environments. 

Some scientists suggest that life may have begun on Earth in hydrothermal vents far below the ocean surface. Environments such as hot oceans, hot springs and deep ocean hydrothermal vents would have been common during the Archaean eon, about 3.9 billion years ago. Early forms of life lived in those conditions.

Types of extremophiles 
Most known extremophiles are microbes. The domain Archaea has well-known examples of extremophiles, but some bacteria are also extremophiles. It is a mistake to use the term extremophile for all archaeans, since some of them are mesophilic. Not all extremophiles are unicellular: some extreme environments have animals which are multicellular. 

Some extremophiles fall under several categories. For example, organisms living inside hot rocks deep under Earth's surface are both thermophilic and barophilic.

 Acidophiles An organism that grows best at pH levels of 3 or below.
 Alkaliphiles An organism that grows best at pH levels of 9 or above
 Endoliths An organism that lives in microscopic spaces within rocks, fissures, aquifers, and faults filled with groundwater deep under the ground.
 Halophiles An organism that needs a high concentration of salt to grow.
 Hyperthermophile An organism that can live well at temperatures between 80–122 °C, as in hydrothermal vents in the deep ocean.
 Hypoliths An organism that lives inside rocks in cold deserts.
 Lithoautotrophs An organism (usually bacteria) that only gets carbon from carbon dioxide and inorganic oxidation. They are chemolithotrophs, such as Nitrosomonas europaea. These organisms get energy from mineral compounds such as iron pyrites. They act in geochemical cycling and wearing away of bedrock to form soil.
 Metalotolerant These organisms are not harmed by high levels of dissolved heavy metals in solution, such as copper, cadmium, arsenic, and zinc.
 Oligotroph An organism that can grow in environments with very few nutrients.
 Osmophile An organism that can grow in environments with lots of sugar.
 Piezophile An organism that lives best under high pressure, such as deep inside the Earth's surface and in deep ocean trenches.
 Polyextremophile An organism that is in more than one category of extremophiles.
 Psychrophile/Cryophile An organism that grows better at temperatures of 15 °C or lower. They are common in cold soils, permafrost, polar ice, cold ocean water, and under snow in high mountains.
 Radioresistant Organisms that can live with high levels of ionizing radiation. Usually this refers to  ultraviolet radiation, but there are also some organisms which can repair damage cause by gamma radiation.
 Thermophile An organism that can live well at temperatures between 60–80 °C.
 Thermoacidophile An organism that is both a thermophile and an acidophile. It grows best in temperatures of 70–80 °C and pH between 2 and 3.
 Xerophile An organism that can grow in extremely dry places, such as the Atacama Desert.

References 

Microbiology

Protista