What Is a Port Scan?

A port scan is a methodical attempt to gain unauthorized access to a computer network. Port scans, usually done with sophisticated software utilities, send messages to individual TCP or UDP ports on a network gateway device. A port scan attempts to identify specific port numbers on the gateway that may be unsecured and thus open to further probing and possible network attack.
→ More - Introduction to Port Scanning (netsecurity.about.com)
→ See also - Can Port Scans Damage Network Switches?