Logging in from other computers
Although SSH has many uses, its main job is to provide access to a command-line over a network. Even if you are installing SSH for completely different reasons, it's best to get command-line access before you try anything more complicated.
From Unix-like systems (including Mac OS X)
Using the command-line
All modern Unix-like systems (Linux, OS X, BSDs, and others) include a command-line ssh client. To login to your computer from a Unix-like machine, go to a command-line and type:
ssh <username>@<computer name or IP address>
You should get the usual password prompt (or be told you can't log in, if passwords are disabled).
See ssh keys if you want to authenticate using keys instead of passwords.
PuTTY is a popular graphical SSH client. You can download the Windows client from here, or the Linux version by installing the following package: putty. Note that you may need to add the Universe repository before installing PuTTY in Ubuntu.
To log in to your computer, type your computer's name or IP address into the "Host Name (or IP address)" box, click on the "SSH" radio button, then click "Open". You will be asked for your username and password, then you'll get a command-line on your Linux computer.
Tip: Keep alive
Your PuTTY (ssh) session will automatically log out if it is idle. To keep the connection active (alive), before you make the connection, select "Connection" on the left of the PuTTY Configuration window, and type 120 in the "Seconds between keepalives (0 to turn off)" box.
Cygwin is a unix-like environment for Windows. You can download it here: Cygwin
To install it, run the Setup.exe and choose a server.
To use ssh, you need to install the OpenSSH package (which includes BOTH a server and client). It is located under "Net" and is called openssh.
Then you can launch Cygwin as a .bat file and get a Terminal Interface:
Now follow the earlier instructions for using the command-line on a Unix-like OS.