Secure Shell (SSH) allows secure communication between networked computers for such purposes as logging in to a remote computer, running some commands remotely, and transferring files (with the scp command).
By default SSH is not enabled in Ubuntu. There is an ssh command installed, but it is only a client, and only allows you to login into another computer, not to allow others to login into yours.
To enable that you first need to install the OpenSSH Server. To do that just use apt-get:
sudo apt-get install openssh-server
If you prefer you can also search for openssh server in the Ubuntu Software Center and install it that way.
Once it is installed you need to enable it in the OpenSSH Server configuration. To do this open and edit the /etc/ssh/ssh_config file with superuser privileges:
sudo nano /etc/ssh/ssh_config
The nano program is a terminal based text editor, but if you prefer a graphical editor you can open it in gedit:
$ sudo gedit /etc/ssh/ssh_config
In that configuration file look for the Port 22 line and uncomment it by removing the preceding hash sign #. That's all you need to edit to get the SSH server working, but if you wish you can review, enable, and edit other configuration options.
Once you're done save the file and restart SSH (which was started automatically when openssh-server was installed) for changes to take effect:
sudo service ssh restart
… or using the old method:
$ sudo /etc/init.d/ssh restart
Your Ubuntu machine will now be able to accept SSH logins and communications through its IP address or host domain.