Welcome to the Java version of Ethereal (or WireShark as they call themselves now)
What is JEthereal?
JEthereal serves many purposes but in short it is a packet inspection program. It looks at packets that are coming and and going out of your comptuer and tells you exactly what they are. Want to see what that program is really sending to Microsoft? Curious about what your computer is doing while the network light is blinking? Someone gave you a pcap dump file and you have no idea what to do with it? Try out JEthereal to solve these problems and many more.
Tell me more
There really isn't much more than that. If you can (and want to) run a signed java applet, go here to try it out: Try JEthereal in an Applet