A Network Security Scanner is a program which attempts to find Security vulnerabilities in one or more systems connected to a network. Normally, the vulnerabilities should be Security Exploit-able by someone who does not have prior authorization to use the target system.
Normally a Network Security Scanner works by testing for available services on a target machine, and then trying out known Security Exploit's against matching services.
The objective of a Network Security Scanner is to find a way to gain unauthorized
access to any of the target systems, or to cause a Denial of Service
on any of those systems.