What makes Netflow collection different from snmp monitoring ?

The key difference resides in the information access: SNMP requires collectors to request the information. Netflow collectors passively receive and process flows from all devices. For first case (polling), devices need to store the data available on request. With Netflow, devices send data once processed. Thus, if devices embed the right processing engines (Deep Packet Inspection, passive probe, …), one could have much more detail on traffic and performance using Netflow.