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.
Features: Choice of the interface to monitoring. Period to monitoring: Day, Week, Month, Year or Custom Days. With autoupdate. Threshold: Autodisconnection if a limit is reached (by NetworkManager). Traffic Monitoring: Inbound, outbount and total traffic; Show the traffic speed. Time Monitoring: Total time of connections in the period. Time Slot Monitoring: Number of sessions used. Reports: Show of average values and daily traffic of a configurable period. Online checking with NetworkManager or by "Ping Mode". The traffic is attributed to the day when the session began. Not need root privilege. Not invasive, use a system try icon.
Now you want to configure your interface
i.e sudo vnstat -u -i eth0
Now to see the usage type vnstat in terminal