eth0 traffic

At some time the traffic on my ADSL connection eth0 is very active although I am doing nothing on the net. Okay, I have Skype running and ntpd is looking after the time but that cannot be the reason for the many exchanges between my computer and the net. The KNemo gives a fair idea what is going on without giving any clue with what it is connecting and what it seems to do exactly. Is there something I can do to find out what kind of traffic is going on. Perhaps somebody who is trying to get access to my computer?