RC4 is a stream cipher encryption algorithm that uses a variable-length key to encrypt data one byte at a time. Developed by Ron Rivest in 1987, RC4 is known for its simplicity and speed, making it widely used in protocols such as SSL/TLS and WEP. However, due to discovered vulnerabilities and weaknesses in its key scheduling and output generation, RC4 has been largely deprecated and replaced by more secure encryption methods. Despite its historical significance, it is no longer recommended for use in modern data security practices.