I can see the data transferred (internet Download/Upload) in the current session from the System Monitor. But I want something that keeps track of all the data transferred and I can see how much data was used in a month or two. Preferably a GUI tool.
If you like a nice GUI try NTM - Network Traffic Monitor. It's not in the repos but you can easily download a deb package from their website: http://netramon.sourceforge.net/eng/index.html
Should work for all *buntu variants.
Now you want to configure your interface
i.e sudo vnstat -u -i eth0
Now to see the usage type vnstat in terminal