The only difference is the security transport / authentication, otherwise if functions the exact same way.
The original RFC for SFTP was written to actually be a simpler version of FTP. While v3 seems to be the standard and the final RFC was never ratified, SFTP is still FTP.
Are you perhaps referring to FTPS, which is FTP with SSL/TLS added? SFTP is very, very different from FTP.
Note that SFTP is not a protocol for security transport / authentication. It doesn't do that. It assumes that you've already secured/authorized the channel with SSH. See section 1 of the RFC.
The original RFC for SFTP was written to actually be a simpler version of FTP. While v3 seems to be the standard and the final RFC was never ratified, SFTP is still FTP.