NetFlow is a network protocol (developed by Cisco) that monitors and collects important network traffic information, including peak usage times, traffic routing and application usage, to show the overall flow and volume of the network. Basically, it can track where traffic is coming from, where it is going and how much traffic is being generated at any time.
This network data can be captured at the device level, using for example, a router with the NetFlow feature enabled. Devices that use a NetFlow collector (hardware or software-based controllers) process the data and present it in readable format. There are a number of NetFlow collectors including Cisco, freeware and third party commercial vendors’ products that report and utilize NetFlow data.
Image: Creating a flow in NetFlow, source; Introduction to Cisco IOS NetFlow
