SSH (Secure Shell)

Secure Shell or SSH is a network protocol allowing secure network communications between two computers, by utilising encryption.

SSH is used primarily for:

• Secure login to a remote server shell, allowing command execution on the server, or access to SCP or SFTP
• Port-forwarding, allowing a computer to connect to services which are behind a firewall, via the SSH server

SSH uses TCP port 22 by default.

Software

• OpenSSH - Opensource implementation of the SSH protocol. OpenSSH is the most commonly used server software for SSH, and most common client software for SSH on Linux.
• PuTTY - Free telnet/SSH client. This is the most commonly used SSH client for Windows based systems.

More info

• Wikipedia Guide to SSH
• How to secure an SSH Server Guide to SSH at BodHost
• SSH FAQ