Web shield is firewall? It protects of invasions?
No, it is not.
Just in case, Network Shield isn’t a firewall either, avast doesn’t contain a firewall.
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). It 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. 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.