SFTP is an abbreviation for SSH File Transfer Protocol, and is exactly that - a protocol for transfering files over an SSH connection.
SFTP is not the standard FTP protocol running over SSH. Although SFTP has similar capabilities and even similar commands to standard FTP, these similarities are purely superficial. The protocol is completely different and incompatible with FTP and its secure extension, FTPS.
SCP is also a file transfer protocol that runs over SSH connections. It is a precursor to SFTP, and allows the copying of files and directories over SSH.
In order to understand SFTP and SCP, it is helpful to have a basic understanding of SSH.
See also:
· SSH - Secure Shell
· SCP - Secure Copy
· SFTP - SSH File Transfer Protocol
· Comparison of FTPS and SFTP