If you are using a smartphone with apps installed,
some of the apps might be using the Internet connection to
update weather information, Facebook news, automatic email download etc.
This internet usage will be charged based on your Internet price plan.
If you want to avoid any Internet sessions running in the background,
you can adjust the setting for each App manually, or disable mobile Internet on your smartphone,
which can be done from Settings menu. You can also disable mobile
Internet by sending a SMS to 500 with "internet off". It is free of charge for sending SMS to 500 to enable or disable the internet connection.
If you wish to re-enable mobile Internet, send an SMS to 500 with ‘internet on’.
You might have to restart your handset for the changes to take effect.