A block cipher is a type of encryption algorithm that transforms data into a secure format by processing fixed-size blocks of plaintext. Each block is typically 64 or 128 bits long, and the same key is used to encrypt and decrypt the data. This method ensures that even if the same plaintext block is encrypted multiple times, the output will differ if a different key is used. Block ciphers are widely used in various security protocols, including AES (Advanced Encryption Standard) and DES (Data Encryption Standard). They provide confidentiality and integrity for sensitive information, making them essential in applications like secure communications and data storage.