Wireshark

Wireshark it is The Ethereal network protocol analyzer with the name changed . The name might be new, but the software is the same. Wiresharks powerful features make it the tool of choice for network troubleshooting, protocol development, and education worldwide. Wireshark was written by networking experts around the world, and is an example of the power of open source. It runs on Windows, Linux, UNIX, and other platforms. This download is for Windows only. Wireshark can read live data from Ethernet, Token-Ring, FDDI, serial (PPP and SLIP) (if the OS on which its running allows Wireshark to do so), 802.11 wireless LAN (if the OS on which its running allows Wireshark to do so) and ATM connections (if the OS on which its running allows Wireshark to do so).

Key features:
>Deep inspection of hundreds of protocols, with more being added all the time
>Live capture and offline analysis
>Multi-platform: Runs on Windows, Linux, OS X, Solaris, FreeBSD, NetBSD, and many others
>Captured network data can be browsed via a GUI, or via the TTY-mode TShark utility
>Read/write many different capture file formats: tcpdump (libpcap), Catapult DCT2000, Cisco Secure IDS iplog, Microsoft Network Monitor, Network General Sniffer® (compressed and uncompressed), Sniffer® Pro, and NetXray®, Network Instruments Observer etc
>Capture files compressed with gzip can be decompressed on the fly
>The most powerful display filters in the industry
>Live data can be read from Ethernet, IEEE 802.11, PPP/HDLC, ATM, Bluetooth, USB, Token Ring, Frame Relay, FDDI, and others (depending on your platfrom)
>Decryption support for many protocols, including IPsec, ISAKMP, Kerberos, SNMPv3, SSL/TLS, WEP, and WPA/WPA2
>Coloring rules can be applied to the packet list for quick, intuitive analysis
>Output can be exported to XML, PostScript®, CSV, or plain text
>Standard three-pane packet browser
>Rich VoIP analysis

A network packet analyzer will try to capture network packets and tries to display that packet data as detailed as possible.
You could think of a network packet analyzer as a measuring device used to examine whats going on inside a network cable, just like a voltmeter is used by an electrician to examine whats going on inside an electric cable (but at a higher level, of course).