Here is a recent update on the Wow! signal, from a popular source. The Wikipedia entry I linked to in the opening post above is more informative, and authoritative.

Incidentally, what’s so important about the 21cm line (1420 MHz). It is the signal given off by monoatomic hydrogen in interstellar space. Think of a single hydrogen atom (NOT the more common molecular species H2) floating in the void. Every now and then, the orbiting electron flying around the nuclear proton will flip its quantum state from positive spin to negative. This releases a single photon of light energy in the microwave region of the spectrum: the neutral hydrogen line of 21 cm wavelength. Its a tiny amount of energy, but there is so much neutral hydrogen in space (perhaps, by mass, the most common normal matter in the galaxy) that it forms the major contribution to natural radio emission. Radio astronomers use it to map the distribution of hydrogen in the spiral arms, since this gas is otherwise invisible. Any civilization with an interest in astronomy would know this, and they would also know that any other species interested in astronomy would be listening in on that frequency. This freq, and its harmonics, which also lie in a part of the radio spectrum which is otherwise nearly static free, is a natural place for civilizations with electronic technology to communicate. It is the Waterhole, the dip between the microwave emission spikes for the H+ and OH- ions.

*λν = c
λ (lambda) = wavelength (in cm)
ν (nu) = frequency (in 1/sec)
c = the speed of light (in cm/sec)