"Under Linux there are GUIs (graphical user interfaces), where you can point and click and drag, and hopefully get work done without first reading lots of documentation. The traditional Unix environment is a CLI (command line interface), where you type commands to tell the computer what to do. That is faster and more powerful, but requires finding out what the commands are."
-- from man intro(1)
Anyway here is the command that allows to logout from your current session.
Open the terminal and type
gnome-session-save --kill --silent
The First command will give you a pop-up showing a logout button and the if you try the second the sytem will logout automatically.