Maybe a non-geek that knows of the existence of a firewall is already some kind of a nerd. If you have been a windows-user then it was rather easy to set up a firewall (I don't know if it was a good one!): enable a firewall with maximum protection. Then for each program that tried to make an internet-connection a notification was shown (allow or not allow). In this way without any prior knowledge of protocols and ports a user could set up a firewall that was suited to his needs. Is there any equivalent program in Ubuntu so that any user can set up a proper firewall?
If you are using a router, and you have not disabled it then you most likely are behind a what they refer to as a "hardware firewall", for most users this is fine...