Network Shield filters traffic coming from all applications (not only browsers), and on all ports. For performance reasons, though, it tries a bit harder in case of the well-known HTTP ports.
Network Shield works on both DNS and HTTP level, i.e., blocks domains on DNS level, but in no way it's limited to whole domains only. The plan is to actually only block malicious URLs unless they're 100% certain there's no useful stuff hosted on the domain (in which case they will block it altogether). It can also block by IP's.
Also, 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.
WebShield scans only http traffic on redirected ports (generally, 80 only). It stops the connection to malware even before the file is saved to the computer.
