From the longer Wikipedia page 
Created in the 15th century (1466/67), it was the peak of cryptography at that time. Its inventor was Leon Battista Alberti , an illegitimate son of an Italian nobleman. He was also interested in painting and writing, though he is probably best known for his architecture. He created the cipher after a conversation on the art of print with Leonardo Dati in the gardens of the Vatican.
The Alberti Cipher Disk described by Leon Battista Alberti in his treatise 'De Cifris' embodies the first example of polyalphabetic substitution with mixed alphabets and variable period. This device, called Formula, is made up of two concentric disks, attached by a common pin, which can rotate one with respect to the other.
The larger one is called Stabilis [stationary or fixed], the smaller one is called Mobilis [movable]. The circumference of each disk is divided into 24 equal cells. The outer ring contains one uppercase alphabet for plaintext and the inner ring has a lowercase mixed alphabet for ciphertext. The outer ring also includes the numbers 1 to 4 for the superencipherment of a codebook containing 336 phrases with assigned numerical values. This is a very effective method of concealing the code-numbers, since their equivalents cannot be distinguished from the other garbled letters. The sliding of the alphabets is controlled by key letters included in the body of the cryptogram.
Compared to previous ciphers of the time the Alberti Cipher was impossible to break without knowledge of the method. This was because the frequency distribution of the letters was masked and frequency analysis - the only known technique for attacking ciphers at that time - was no help
See also the Wikipedia page on the Alberti cipher disk