Cryptography is a way to ensure your information is kept secure from prying eyes. Cryptography ranges from weak to unbreakable with varying amounts of effort required. Examples of a weak codes is a simple substitution cipher where letters and numbers are replaced one for one with others. At the other end of the spectrum is a one time pad code: if the attacker has access to the encrypted data stream only, this type of code is absolutely unbreakable no matter what resources (supercomputers etc) are available.
Paper And Pen Based Codes
As long as the code is never reused, the paper code books are kept secure, the numbers are truly randomly generated (a pair of dice), the pages are fully destroyed after use and the humans on either end of the message are not tortured, the one time pad is the ultimate in secure messaging.