The Common Internet File System (CIFS) is a protocol that allows file sharing across networks. It was originally developed by Microsoft as an extension of the Server Message Block (SMB) protocol to enable file sharing between DOS and Windows machines. CIFS has become a widely adopted standard, allowing file sharing between Windows, Mac, and Linux machines. It provides methods for accessing files over networks, along with features like file locking to prevent conflicts when multiple users access the same file simultaneously. CIFS is still commonly used today for file sharing within organizations and across the internet.