SFTP (SSH File Transfer Protocol), not to be confused with FTPS (Secure FTP), runs on top of the SSH (Secure Shell) protocol and by default uses port 22 for communications. An SFTP server may however be configured to listen on a different port other than the default port. Listening on a port other than the default port is a common tactic used to reduce the number of attacks, in particular scripted brute-force password attacks.
Unlike FTP/S protocols which use separate channels (i.e. ports) for exchanging data and commands,the SFTP protocol uses a single encrypted channel to exchange all data. The fact that SFTP uses a single port for all communications can make it easier to use than FTP/S when behind firewalls with strict policies. For additional information see the article Understanding Key Differences Between FTP, FTPS and SFTP.