Codes have been around for a very long time. Their general intent has typically been linked to warfare and includes secret discemination of information to authorized parties while making it difficult or impossible for non-authorized parties to gain access to the information, even if they intercept the coded message. So that we can discuss this topic, a few definitions are in order:

Ciphers

Ciphers are the oldest and simplest to understand. Ciphers take one character of a source document and convert it to a different character in the destination document. It can do this by converting one character into another, by moving the characters around within the document, or by a combination of both.

Codes

Codes are a logical extension of ciphers in that whole words, phrases, dates, etc. are converted to a new sequence in the encrypted text.

Decryption

Decryption is the process of converting coded text into the original plain text.

Encryption

Encryption is the process whereby plain text is convered into an encoded (or encrypted) text.

In today's modern world, many reasons exist for encryption. Internet security is a large concern to anybody who uses the internet for commerce. Security of intellectual property (i.e. keeping Trade Secrets) is the basis for the financial solvency of a number of companies.

Unfortunately, in our modern world, keeping secrets is becoming more and more difficult. As computers gain more and more popularity, the ability of the general person to keep their own personal information, let alone any electronic conversations, secret is both more difficult and easier than ever before. More difficult in that more individuals have access to the data while it's being transmitted from source to destination. Easier in that the tedium necessary to implement any cipher is easily accomplished by todays computers.

Several "simple" cipher and code systems are presented below. Each provides a limited example encoder/decoder for your education.

Character Shift

Rail Fence

Play Fair

Jefferson Wheel Cipher

Public Key Ciphers

Enigma Cipher

One Time Pad

A Cryptographic Compendium is the home of a very extensive coverage of the topic.

Wiretapped.net maintains an archive of source code implementations of various cryptographic techniques thats quite interesting.

Clement Dupuis maintains "A Short History of Crypto" that is an interesting historical prespective.

NOVA aired a story about various classic cryptographic techniques (I'd love to catch this sometime).

Sci.Crypt FAQ holds an astonishingly large amount of information about the field.