A block cipher is a method of encrypting data in fixed-size blocks, typically 64 or 128 bits. It transforms plaintext into ciphertext using a secret key, ensuring that only authorized users can access the original information. This process involves various algorithms that apply mathematical functions to the data, making it difficult for unauthorized parties to decipher. Block ciphers are widely used in various security protocols, including AES (Advanced Encryption Standard) and DES (Data Encryption Standard). They are essential for protecting sensitive information in applications like online banking, secure communications, and data storage, providing a robust layer of security against potential threats.