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 small changes in the input result in significantly different outputs, enhancing security. Block ciphers use various techniques, such as substitution and permutation, to scramble the data within each block. Popular examples include AES (Advanced Encryption Standard) and DES (Data Encryption Standard). These algorithms are widely used in securing data for applications like file encryption and secure communications.