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 transform plain text into a scrambled format, ensuring that only authorized individuals can access the original message. They are widely used in various fields, including computer security and military communications. There are many types of ciphers, such as Caesar cipher, which shifts letters by a fixed number, and Vigenère cipher, which uses a keyword to determine the shift for each letter. Ciphers play a crucial role in protecting sensitive information from unauthorized access.