Packets are the smallest units of information sent across networks. A single packet contains one piece of data—such as an IP address, TCP port number, or HTTP request header—that travels over multiple physical connections. Each packet typically takes up less than a kilobyte of memory, making it easy to store and transmit.

A packet capture tool allows you to monitor and record packets as they travel across a computer network. This lets you see what’s happening behind the scenes of your organization’s network infrastructure. You could even use a packet capture tool on your home network to analyze bandwidth.

The most basic form of packet capture involves running software on a device that monitors network traffic and saves captured packets into a file. These files are called “packet captures.” They’re usually stored in the .pcap file extension.

More info: Active Vs Passive Monitoring