SFTP protocol is almost always layered on top of an SSH channel, but the protocol itself was designed to be transport layer independent. Its packet-based nature makes it perfectly suitable to be layered on top of WebSockets protocol, which provides one major benefit over classic SFTP over SSH: It can be easily used from browser-based JavaScript web applications. SFTP over WebSockets (let's call it SFTP.WS) makes it possible to implement a fully-features SFTP client running in a browser that can directly communicate with any SFTP.WS-enabled SFTP server, eliminating the need for proptietary and unreliable AJAX-based solutions.
This is work in progress. Please stay tuned for additional information.