network sniffer - Definition from the Hotspot Shield Glossary
A network sniffer is used to monitor data across a network in a real-time session. Network Sniffers can also be called Snoops, Probes, or Packet Sniffers but all perform the same general tasks. A Network Sniffer will either be a software program or a piece of hardware that takes “snapshots” of network data but does not actually altar said data or redirect it in any way.
Historically, Network Sniffers were used by Network engineers to identify how protocols worked and how a network was dealing with the flow of traffic, however today they are more popular with hackers and individuals who are just researching networking. While sniffer tools are a fantastic way to analyze network traffic, they can also give easy access to potentially sensitive data.