Ethereal is a programm that listens to the network traffic.
There are capture filters for example:
to see all the traffic comming and going to that host.
Attention, capture filters are different from display filters: an example of a display filter is
You'll see only the http traffic --> cool! you can see the user agent, the get requests and the sites visited by a particular host.
After capturing network trafic, you can save the result to a cap file.