A block cipher is a type of encryption algorithm that transforms data into a fixed-size block, typically 64 or 128 bits, using a secret key. It processes the data in chunks, ensuring that each block is encrypted independently. This method enhances security by making it difficult for unauthorized users to decipher the information without the key. Block ciphers are widely used in various applications, including secure communications and data storage. Popular examples of block ciphers include AES (Advanced Encryption Standard) and DES (Data Encryption Standard). These algorithms are essential for protecting sensitive information in digital systems.