A Feistel network is a structure used in the design of symmetric key block ciphers. It divides the input data into two halves and processes them through multiple rounds of encryption. Each round involves a function that mixes the data, ensuring that small changes in the input lead to significant changes in the output. The key feature of a Feistel network is that it allows for the decryption process to be the same as the encryption process, simply by reversing the order of the keys used. This makes it efficient and practical for creating secure encryption algorithms, such as DES (Data Encryption Standard).