We'll point you in the right direction, but not tell you how. :D
The information is called packets. The technique has the word sniffing in it. There are several good programs out there for doing it, they're referred to as sniffers.
But viewing other people's traffic without permission is unethical.
Ethical ways to do it:
1) work on a network, where the employ handbook says to not expect privacy and or traffic is monitored, and have an email from upper management over IT send you an email asking you to do it.
2) Own (as in monetarily) a home network with the same level of privacy as above (ie none), and monitor the inbound and outbound traffic.
3) Fire up a monitor / sniffer, and an IRC channel. Watch irc from the sniffer. (Don't laugh it's how I learned to work with the sniffers at home, and was fairly fun).
Catch a cheating spouse
spy on co-workers
spy on anyone
try to get black mail material
trying to get information you're not supposed to have.