A program that queries every TCP
port on target host
and lists all open
ports. Used to see what network services
the target host is running (probably in order to see if the host has exploit
able security holes
These can be used for legal purposes too - for example, to check what ports you have open to outside. (In other words: You can do this unto you before someone else gets the same idea. =)
One of the best is called nmap. It also scans UDP and does all sorts of other things.