I work for a network security company but in a non security position, and I don't have my security clearance yet so I don't know exactly all the security stuff what do, but I know we do network security audits and penetration test to let our clients know where the weakpoints in their networks are.

This is done with some software tools, I think Nessus and nmap are two of the ones we use. So just to give you just one possibility of what a job in network security might be.