You see encryption is everywhere,when you watch some spy film you see there's always a computer with encrypted files or some film about hackers when the feds busted the hacker and they see all of the hacker's files are encrypted. But when you use encryption and encrypt the file it will become unreadable by the time you don't enter the password. This text is called cipher text. The process of converting a cipher text into plain text is called decryption. This example shows you the way when you encrypt and decrypt a file.
Cryptography is science that use the mathematics to encrypt and decrypt data.
This science let you keep your files and documents safe even on insecure networks like the Internet. The cryptography can be weak and strong. The best is of course the strong one. This saves you disk space and modem transmition. Then it creates a session key.