FAQ: Firewall Forensics (What am I seeing?):
"This document explains what you see in firewall logs, especially what port numbers means. You can use this information to help figure out what hackers are up to.

This document is intended for both security-experts maintaining corporate firewalls as well as home users of personal firewalls. "

http://www.secinf.net/firewalls_and_VPN/FAQ_Firewall_Forensics_What_am_I_seeing_.html