The protocol commonly used for transmitting files over the internet is File Transfer Protocol (FTP). FTP allows users to upload and download files between a client and server. It supports various file types and can be run in active or passive modes, making it versatile for different network configurations. FTP is essential for web development, data backup, and file sharing, facilitating efficient and secure data transfers. Users may utilize FTP clients or command-line tools to connect to FTP servers, ensuring data integrity during transmission.