Once in a while a friend, family member, or colleague may want to borrow your computer. The Ubuntu Guest Session feature provides a convenient way, with a high level of security, to lend your computer to someone else. A guest session can be launched either from the login screen or from within a regular session. If you are currently logged in, click the icon at the far right of the menu bar and select Guest Session. This will lock the screen for your own session and start the guest session.

A guest cannot view the home folders of other users, and by default any saved data or changed settings will be removed/reset at logout. It means that each session starts with a fresh environment, unaffected by what previous guests did.

The Customize Guest Session online tutorial explains how to customize the appearance and behavior.

If you prefer to not allow guest access to your computer, you can disable the Guest Session feature. To do so, press Ctrl+Alt+T to open a terminal window, and then run this command (it's one long command, even if it may be shown wrapped on the screen - copy and paste to get it right):

sudo sh -c 'printf "[SeatDefaults]\nallow-guest=false\n" >/usr/share/lightdm/lightdm.conf.d/50-no-guest.conf'

The command creates a small configuration file. To re-enable Guest Session, simply remove that file:

sudo rm /usr/share/lightdm/lightdm.conf.d/50-no-guest.conf