A cipher is a method used to transform information into a format that is unreadable to anyone who does not have the key to decode it. This process involves substituting or rearranging letters and symbols to create a secret message. Ciphers are commonly used in various fields, including computer security and military communications, to protect sensitive information. There are many types of ciphers, such as the Caesar cipher, which shifts letters by a fixed number, and the Vigenère cipher, which uses a keyword to determine the shift for each letter. Understanding ciphers is essential for cryptography, the study of secure communication techniques.