Counting Network Traffic enables you to track your Internet bandwidth usage, the 3G, 2G network traffic which is being consumed in your mobile phones and computers.
If you are using a iPhone, there is no need to install any additional options. iPhone has a inbuilt option to track your mobile network data usage. This feature in iPhone is called Cellular Network Data Counter.
You can change the settings to enable this feature. Just go to Settings -> General -> Usage and Enable the Cellular Usage.
With this, you can know the amount of data sent and received up to date. And if you are looking for a separate iPhone application with this feature, just go to iTunes and search for Data Usage Monitor. This free application will alert you when your plan limit is about to end. You can also know the area of location through which your network is accessed.