:: What is ssh
Secure Shell or SSH is a network protocol that allows data to be exchanged using a secure channel between two networked devices. The two major versions of the protocol are referred to as SSH1 or SSH-1 and SSH2 or SSH-2. 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. The encryption used by SSH is intended to provide confidentiality and integrity of data over an unsecured network, such as the Internet. :: wikipedia

 
:: How to setup ssh server on Debian

# aptitude install openssh-server

 
:: Checking ssd

# /etc/init.d/ssh status
or
# netstat -tlupn | grep 22
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      5662/sshd
tcp6       0      0 :::22                   :::*                    LISTEN      5662/sshd

 
:: Start / Restart sshd

# /etc/init.d/ssh start
or
# /etc/init.d/ssh restart

 
:: Testing sshd

# ssh localhost
or
# ssh username@localhost