A packet sniffer is a tool used to monitor and analyze network traffic. It captures data packets that travel across a network, allowing users to inspect the contents of these packets. This can help in troubleshooting network issues, monitoring performance, and ensuring security by detecting unauthorized access or malicious activity. Packet sniffers can be both hardware devices and software applications. They operate by placing the network interface into a mode that allows it to capture all packets, not just those intended for the device. Commonly used packet sniffers include tools like Wireshark and tcpdump, which provide detailed insights into network communications.