Ubuntu has introduced the Guest Session since the release of Oneiric Ocelot. The guest session allow any user with physical access to your computer to do some basic tasks such as browsing the Internet. This can be very helpful in case you want to lend your computer to your friend or colleague.

However, some users may have concerns about the security issues of this feature but there is no need to worry as this session runs in a restricted environment with a high level of security so the Guest Session user cannot access your files or informations.
Still not convinced ?

OK, then i will show you how to disable it:

First, you need to edit the LightDm configuration file using this command:

gksudo gedit /etc/lightdm/lightdm.conf
After typing this command in a Terminal, a gedit window will open. To disable the guest session, you need to add this line allow-guest=false to the end of the file. After this modification, your lightdm.conf file should look something like this: