OpenSSH Daemon, or sshd is the server side of ssh secure communications. It listens out for incoming connections from ssh clients and spawns a process to handle them. You can install sshd either directly using apt-get, or with tasksel (a handy graphical software installation interface).
Ubuntu server comes with tasksel installed by default, the desktop version does not. If you need to install tasksel, just run:
sudo apt-get install tasksel
To use tasksel to install sshd, follow these steps:
Navigate the cursor to OpenSSH server, then hit the space key to select it.
Hit the return key
Installation using apt-get
To install sshd using apt-get, run:
sudo apt-get install openssh-server
If you have a firewall enabled (and you really should!), you'll need to ensure it is allowing ssh traffic through. By default sshd listens for incoming connections on port 22 (If you've changed this port number use the new number below). To allow traffic to reach this port, run:
sudo ufw allow 22
Now you'll be able to ssh to the box from another box.