Despite SSH in its name, it is designed to work over any reliable data stream, but WinSCP supports only operation over SSH, which is also by far its most common usage.
Being operated over SSH, it is secure protocol. In its basic functionality it is similar to old FTP, while having better designed advanced functionality. Unfortunately not all SFTP server implementations take advantage of the advanced features, yet.
Unlike SCP, for connection with an SFTP server you do not need access to shell (although some implementations may still require that).
See also SFTP status/error codes.
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