It was an electro-mechanic cryptographic machine used to encrypt and decrypt secret messages. Famous, though, for being used by the German military forces during the 1930's. Its ease of use and alleged indecipherability of its codes were the main reasons for its popularity. The code, however, was deciphered, and the information in the messages it protected is generally thought of as responsable for the end of World War II at least one year earlier than most people predicted.
~Ally