Superfluid helium is what you get when you cool helium to near
absolute zero under normal atmospheric pressure (1 atm). From
absolute zero up, the phases of helium at normal atmospheric
pressure are superfluid, liquid, gas; higher pressure than
atmospheric is required to produce solid helium. Superfluid is a
state of matter that can only be understood using quantum
mechanics. It is similar to a liquid, but different in certain
ways: in particular, zero viscosity and infinite thermal
conductivity. It is related to a Bose-Einstein Condensate. The term
superfluid helium usually refers to superfluid ^4Helium, helium-4
with a nucleus containing two protons and two neutrons and overall
quantum mechanical spin zero. Superfluid ^4He exists below about 4
degrees Kelvin (at 1 atm). The isotope ^3He, helium with a nucleus
with only one neutron, exists as a superfluid at lower
temperatures, below one kelvin, and different mathematics is
required to describe it; the differences follow from the fact that
the ^3He atom has quantum mechanical spin 1/2.