Traffic vs Data Tutorial
Think of the internet as a road carrying cargo: vehicles are 'packets' and the cargo is your 'data'. The entire group of vehicles and packets constitutes 'traffic'.
Some vehicles, like semi-trailers, carry large amounts of data with a small percentage of overhead; others, like small trucks, carry less data with a larger percentage of overhead. Yet others, like police cars, carry no data and serve special purposes.
Most traffic metering systems measure the traffic passing through the system, while others (especially proxies and mirrors) measure the cargo. This discrepancy can lead to vastly different values between the two systems.