Secure Shell, or SSH, is not only a program, it is also a protocol. It allows for secure, encrypted network connections to a remote computer. SSH is primarily used to log in to remote machines. The newest version, SSH-V6, offers additional functions such as file transfers via SFTP (Secure File Transfer Program). For further information, read SSH on Wikipedia
The Internet Assigned Numbers Authority IANA has allocated port 22 to the SSH protocol, but IPFire strongly recommends running SSH on port 222 because of security reasons.