Secure Shell (SSH) is a protocol primarily for encrypted shell connections. This tag is also used for questions about sshd and openssh, the two standard applications for using SSH.
Secure Shell or SSH is a network protocol that allows data to be exchanged using a secure channel between two networked devices. Used primarily on Linux and Unix based systems to access shell accounts, SSH was designed as a replacement for Telnet and other insecure remote shells, which send information, notably passwords, in plaintext, rendering them susceptible to packet analysis.
It's important to note that there are two versions of SSH (1 and 2), and that version 1 is no longer considered secure, and should be replaced by version 2 where ever possible.
More information, including the more notable SSH-1 vulnerabilities, can be found at the Wikipedia page for SSH.