Tcpdump displays out the headers of packets on a network interface that match the given criteria. You can dump traffic on a network and analysis problems.
This guide provides some good examples:
Tcpdump is the premier network analysis tool for information security and networking enthusiasts and/or professionals. In my own primer I cover tcpdump basics; if you're interested in becoming familiar with the application via an introduction, I suggest you check it out first.
Here I'm simply going to give a number of recipes that you're likely to find useful during your day to day activities. They will range from common, general captures to complex filters designed to look for a number of unique traffic types.