A cipher is a method used to encrypt or encode information, making it unreadable to anyone who does not have the key to decode it. Ciphers can be simple, like substituting one letter for another, or complex, involving mathematical algorithms. They are commonly used in computer security, communications, and data protection to ensure that sensitive information remains confidential. Historically, ciphers have played a significant role in military and diplomatic communications. Famous examples include the Caesar cipher, named after Julius Caesar, and the Enigma machine, used by the Germans during World War II. Understanding ciphers is essential for cryptography, the broader field that studies secure communication techniques.