When you log into a Pi and open a terminal window, or you boot to the command line instead of the graphical user interface, you start in your home folder; this is located at
/home/pi, assuming your username is
This is where the user's own files are kept. The contents of the user's desktop is in a directory here called
Desktop, along with other files and folders.
To navigate to your home folder on the command line, simply type
cd and press
Enter. This is the equivalent of typing
cd /home/pi, where
pi is your username. You can also use the tilde key (
~), for example
cd ~, which can be used to relatively link back to your home folder. For instance,
cd ~/Desktop/ is the same as
/home/ and run
ls, and you'll see the home folders of each of the users on the system.
Note that if logged in as the root user, typing
cd ~ will take you to the root user's home directory; unlike normal users, this is located at
/home/root/. Read more about the root user.
If you have files you would not like to lose, you may want to back up your home folder. Read more about backing up.