There are two kinds of cryptosystems: symmetric and asymmetric. Symmetric cryptosystems use the same key (the secret key) to encrypt and decrypt a message, and asymmetric cryptosystems use one key (the public key) to encrypt a message and a different key (the private key) to decrypt it. Assymetric cryptosystems are also called public key cryptosystems.
Symmetric cryptosystems have a problem: how do you transport the secret key from the sender to the recipient securely and in a tamperproof fashion? If you could send the secret key securely, then, in theory, you wouldn't need the symmetric cryptosystem in the first place -- because you would simply use that secure channel to send your message. Frequently, trusted couriers are used as a solution to this problem. Another, more efficient and reliable solution is a public key cryptosystem, such as RSA, which is used in the popular security tool PGP.
Bruce Schneier's Applied Cryptography (2nd Ed.) is the crypto Bible for the professional engineer and interested layman. It's a good survey of the state of the art in crypto techniques and protocols. You can also subscribe to and read back issues of Crypto-Gram, Bruce Schneier's monthly newsletter about cryptography.
sci.crypt and sci.crypt.research are two USENET newsgroups that deal exclusively with cryptography. The latter is a moderated group.
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