A superfluid is a phase of matter capable of flowing endlessly without energy loss. This property of certain isotopes was discovered by Pyotr Leonidovich Kapitsa, John F. Allen, and Don Misener in 1937. It has been achieved at very low temperatures with at least two isotopes of helium, one isotope of rubidium, and one isotope of lithium.
Only liquids and gasses can be superfluids. For example, helium's freezing point is 1 K (Kelvin) and 25 atmospheres of pressure, the lowest of any element, but the substance begins exhibiting superfluid properties at about 2 K. The phase transition occurs when all the constituent atoms of a sample begin to occupy the same quantum state. This happens when the atoms are placed very close together and cooled down so much that their quantum wave functions begin to overlap and the atoms lose their individual identities, behaving more like a single super-atom than an agglomeration of atoms.
A limiting factor on which materials can exhibit superfluidity and which cannot is that the material must be very very cold (less than 4 K) and remain fluid at this cold temperature. Materials that become solid at low temperatures cannot assume this phase. When cooled to very low temperatures, a superfluid-ready set of bosons, atoms with an even number of nucleons, forms into a Bose-Einstein condensate, a superfluid phase of matter. When fermions, atoms with an odd number of nucleons such as the helium-3 isotope, are cooled down to a few Kelvin, this is not sufficient to cause this transition.
Because only bosons can readily become a Bose-Einstein condensate, fermions must first pair up with each other in order to become a superfluid. This process is similar to the Cooper pairing of electrons that occurs in superconductors. When two atoms with odd numbers of nucleons pair up with each other, they collectively possess an even number of nucleons and begin to behave like bosons, condensing together into a superfluid state. This is called a fermion condensate, and emerges only at the mK (milliKelvin) temperature level rather than at a few Kelvins. The key difference between atom pairing in a superfluid and electron pairing in a superconductor is that the atomic pairing is mediated by quantum spin fluctuations rather than by phonon (vibratory energy) exchange.
Superfluids have some impressive and unique properties that distinguish them from other forms of matter. Because they have no internal viscosity, a vortex formed within one persists forever. A superfluid has zero thermodynamic entropy and infinite thermal conductivity, meaning that no temperature differential can exist between two superfluids or two parts of the same one. They can also climb up and out of a container in a one-atom-thick layer if the container is not sealed. A conventional molecule embedded within a superfluid can move with full rotational freedom, behaving like a gas. Other interesting properties may be discovered in the future.
Most so-called superfluids are not pure, but are in fact a mixture of a fluid component and a superfluid component. The potential applications of superfluids are not as exciting and wide-ranging as those of superconductors, but dilution refrigerators and spectroscopy are two areas where they have found use. Perhaps the most interesting application today is purely educational, showing how quantum effects can become macroscopic in scale under certain extreme conditions.