Phazon is an element in the Metroid series that first debuted in Metroid Prime. It is a substance that was first found and harvested by the Space Pirates that seems to make any living beings (that can withstand its radiation without dying) more powerful and mutagenic. So far, it has only appeared in the Prime series of Metroid games, and will probably stay that way seeing that every known bit of it was eradicated in the final game in the series — Metroid Prime 3: Corruption for the Wii.
However, despite this, it's also a very dangerous and potentially fatal element at direct or close contact. It is well known that it's killed many civilizations and have also corrupted the four bounty hunters Rundas, Ghor, Gandrayda, and of course Samus Aran. The former three all became controlled by Dark Samus and attempted to murder Samus, who ended up being victorious in the fights, but was also forced to kill them all. Phazon seems to be incredibly efficient as fuel (it surpasses Fuel Gel by far), or as an energy source. Prolonged exposure degenerates cerebral tissue and can cause insanity (called 'Phazon madness' by Space Pirates). Phazon infection's capabilities go have a wide range, as they go from resurrection (Dark Samus), to animation of inert objects (Thardus).
Whomever becomes infected with phazon (or posesses a Phazon tank attached) is able to use a PED (Phazon Enhancement Device), made by the Galactic Federation (who stole the plans of it from the Space Pirates) which provides a major boost to their weapons and armor. The PED requires Phazon as fuel, which is drained from the tank. However, Phazon-infected creatures don't require a tank (heavily infected ones don't need a PED at all), as their bodies produce the needed Phazon. This is a potential corruption source, as it overreacts after several seconds, and can prove fatal or extremely virulent if it is not vented out of the user.
Phazon presents many biological characteristics and has a unique way of 'reproducing': a biological creature called a Leviathan. This enormous being is launched into space by Phaaze, the source of all Phazon, after it finds a planet to corrupt. Upon contact, the Leviathan moves towards the target planet's core, slowly corrupting it. All bioforms that reject it are killed by its radiation, the rest become corrupted. On early corruption stages, the Planet is seemingly normal, but with Phazon growing in some areas like caves or pools of liquid Phazon appearing on certain locations, on more advanced stages, blue veins of Phazon (see picture) start protruding out of the Planet's surface (i.e: Pirate Homeworld), until it becomes 100% Phazon.