There are two kinds of particles in the world: bosons and fermions. Bosons have integral spin, and follow Bose statistics. Fermions have half-integral spin, and follow Fermi statistics.
Fermions obey the so-called Pauli exclusion principle. It is impossible for two fermions to occupy the exact same quantum mechanical state. If you have one fermion in a state, the amplitude to put another particle in the same state is exactly zero.
Bosons, on the other hand, do not obey any exlcusion principle. If you have one boson in a state, the amplitude to put another in the same state is non-zero. Futhermore, the amplitude to put another boson into the same state already occupied by two bosons is even larger. In general, bosons "try" to occupy the same state.
Two familiar examples: electrons are fermions. The exclusion principle they follow in atoms leads to the periodic table and all the rest of chemistry and biology. Photons are bosons. You can put as many photons into the same state as you want. Lasers do this, with great numbers of photons.
Now -- what is BCS condensation? It's a mechanism by which fermions can pretend they're bosons by grouping up. The classic example is liquid helium-3. A helium-3 atom is a fermion. When you reduce the temperatures to very close to absolute zero, the helium atoms begin to interact with one another, and form loosely-associated pairs. (At high temperatures, these pairs are disrupted by thermal excitation.) A pair of fermions (each of half-integral spin) acts like a boson (of integral spin). In effect, the two helium atoms act together as a single particle, a boson.
As you decrease the temperature of He-3, all of a sudden you cross a critical point, and the atoms undergo Bose-Einstein condensation. Suddenly, rather than being unable to be in the same state, the pairs seek strongly to be in the same state. In fact, they quickly all enter the same state. This is the "condensation."
Once condensed, the atoms will try very hard to stay that way. The condensed, so-called superfluid, helium flows without viscosity, can climb up the side of containers, and flow easily through the tiniest pores in a container. Why? Because in order to the change the quantum-mechanical state of one helium atom-pair, you have to change the state of all of them. This takes a great deal of energy. Normally, fluids have viscosity because collisions with the walls knocks the fluid particles around willy-nilly. Superfluid helium simply doesn't allow this -- the atoms are so strongly interested in being in the same state that they just don't interact with the walls at all.
Does this help?
- Warren