It is easy for living creatures to survive and grow in some places. Other places make it tough for life to get by. Scientists call places that are hard on life "extreme environments". There is a special name for creatures that can live in extreme environments. They are called "extremophiles".
Most extremophiles are tiny microbes. There are also a few types of slightly larger creatures which are extremophiles. Some kinds of shrimp and insects are extremophiles, for example. In recent years scientists have found extremophiles living in places where they used to think life was impossible. Some extremophiles live in very hot or very cold environments. Others live in environments that have acids or radiation in them.
When Earth was young, most places on our planet were extreme environments. Environments on many other planets and moons are also extreme. Scientists hope that studying extremophiles will help us learn about early life on Earth and about evolution. They also hope we can learn about the chances for life on other planets by studying extremophiles. Scientists who study these sorts of things are called astrobiologists.
Scientists use special names for extremophiles that live in special places. Thermophiles live in hot places. Acidophiles can put up with strong acids. Xerophiles live in deserts and other dry places. Halophiles like it salty. Cryophiles (also called psychrophiles) love the cold. Some creatures are extreme in more than one way. Microbes that live in acidic hot springs are both thermophiles and acidophiles.
A few big big animals and plants also survive in really extreme environments. While they aren't exactly extremophiles, some of them do live in places that would kill most normal creatures. Camels can go for long periods without water in the dry deserts where they live. Emperor penguins somehow make it through cold Antarctic winters. Extreme plants, such as many types of cactus, also live in the heat and dryness of deserts. Strange tube worms grow in boiling hot water filled with strange chemicals near deep sea hydrothermal vents.