"While classical cryptography employs various mathematical techniques to
restrict eavesdroppers from learning the contents of encrypted messages,
in quantum mechanics the information is protected by the laws of physics.
In classical cryptography an absolute security of information cannot be
guaranteed. The Heisenberg uncertainty principle and quantum entanglement
can be exploited in a system of secure communication, often referred to as
'quantum cryptography'. Quantum cryptography provides means for two
parties to exchange a enciphering key over a private channel with complete
security of communication." Artur Ekert

The best overviews of Quantum Cryptography I have seen so far are in two magazine articles recommended by Bruce Schneier. Schneier's book, Applied Cryptography, itself contains a smaller, more concise overview of QC. Also quite worthy of note is a paper by Artur Ekert, one of the early formulators of today's Quantum Cryptography. Soon to be added to this list of admirable literature will be my own research paper, ;), Fast Forward: Quantum Cryptography, written for Darrell's class.  
Charles H. Bennet, Gilles Brassard and Artur K. Ekert,
"Quantum Cryptography", Scientific American,
v. 267, n. 4, Oct 1992, pp. 5057
 
C. Zimmer, "Perfect Gibberish", Discover,
v. 13, n. 12, Dec 1992, pp. 9299
 
Bruce Schneier, Applied Cryptography, John Wiley & Sons,
1996, ch. 23.16
 
Artur Ekert, "What is Quantum Cryptography",
http://eve.physics.ox.ac.uk/NewWeb/Research/crypto.html
 
As an aid to understanding how quantum cryptography works, I wrote a program that would simulate a quantum cryptosystem based on two noncommuting observables  cryptosystem A as mentioned in the paper by Ekert. 