noODle recently wrote a very good tutorial on Packet Sniffing which I recommend (See Thread ), but while doing research for my Computer Security 101: Lesson 5 article I came across this web site with a great introduction to packet sniffing.
It provides an overview of the concepts and some scenarios to explain what you can and can't accomplish with packet sniffing. It has information and links to download a number of different packet sniffing applications including AnalogX Packetman and Ethereal among others.
It also has a link to a PDF file called "Crash Course" to Packet Sniffing and a zip file containing sample data you can view using the packet sniffing tools.
For someone looking to start from scratch and get a decent understanding of packet sniffing this seems like a promising site.
Here is the link: Packet Sniffing