Network Shield is a protection against known Internet worms/attacks. It analyses all network traffic and scans it for malicious contents. It can be also taken as a lightweight firewall (or more precisely, an IDS (Intrusion Detection System).
Network Shield protects you from internet worms that spread themselves via various security holes in your system. Typicaly these kind of viruses don't infect files but instead they attack running processes on your PC (either Windows components or some server programs like SQL Server, IIS etc.). These kind of attacks are not easily catched by ordinary antivirus during file or mail scanning. It is not a duplicate work with Standard Shield.
Basically, it covers all Internet worms. Such as Win32.CodeRed, Win32.SQLSlammer, Win32.Blaster, in32.Welchia (Nachi) and Win32.Sasser.
If you have Windows updated and a firewall, you won't see Network Shield scanning (most probably).