You can use SSH to connect to your Raspberry Pi from a Linux computer or Mac (or another Pi) from the Terminal, without installing additional software.
This is really useful if you want to run your Pi as a 'headless' machine without the need for its own monitor, mouse and keyboard - instead you can use your home computer, ipad or even mobile phone to access and control your Pi.
Step 1: Find Your Pi's IP Address
You'll need to know your Pi's IP address to connect to it.
To find this from your Pi's terminal
type hostname -I
Step 2: Connect Using Pi's IP Address
To connect to your Pi from a different computer, copy and paste the following command into the terminal window but replace with the IP address of the Raspberry Pi.
You can use Ctrl + Shift + V to paste in the terminal.
ssh pi@<Pi's IP address>
If you receive a 'connection timed out' error it's likely that you've entered the wrong IP address for the Raspberry Pi.
Step 3: Connect to the Pi
When the connection works you'll see a security/authenticity warning. Type yes to continue. You'll only see this warning the first time you connect.
In the event your Pi has taken the IP address of a device to which your computer has connected before (even on another network), you may be given a warning and asked to clear the record from your list of known devices. Following this instruction and trying the ssh command again should be successful.
Step 4: Login to Raspberry Pi
Next you'll be prompted for the password for the pi login, by default on Raspbian the password is raspberry.
You should now have the Raspberry Pi prompt which will be identical to the one found on the Raspberry Pi itself.
If you have set up another user on the Pi, you can connect to it the same way, replacing the username with your own, e.g. firstname.lastname@example.org
pi@raspberrypi ~ $
You are now connected to the Pi remotely and can execute commands.
For further documentation on the ssh command just enter man ssh into the Terminal.