A Feistel Network is a symmetric structure used in block ciphers for encryption and decryption. It divides the input data into two halves and processes them through multiple rounds of transformation, where one half is modified based on the other half and a round function. This design allows for the same algorithm to be used for both encryption and decryption, making it efficient. The security of a Feistel Network relies on the complexity of the round function and the number of rounds used. Popular encryption algorithms, such as DES (Data Encryption Standard), utilize this structure, demonstrating its effectiveness in securing data against unauthorized access.