The cipher disk was invented in 1467 by Leon Battista Alberti, a famous Italian philosopher and architect.
Alberti used two different alphabets located on concentric rings - this means one ring is inside of or on top of another. By lining up two different letters, one from each ring, he could make a simple substitution alphabet in which he could create a cipher.
For example, if he aligned the A on the outer ring with the G on the inner ring, this would make the following substitution alphabet used to encrypt a message:
OUTER RING: ABCDEFGHIJKLMNOPQRSTUVWXYZ
INNER RING: GHIJKLMNOPQRSTUVWXYZABCDEF
From there, he could encrypt his message and send it to someone who knew the secret to revealing the message.
http://www.nsa.gov/kids/ciphers/ciphe00005.shtml
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