In a building, a structural firewall prevents damage from propagating across the entire building. It holds potential danger at bay and limits exposure. In a computer the firewall prevents data from going where it should not go.

In a simple application, the firewall prevents information that was not requested from coming into your computer. If you type the address for FoxNews.com, for example, data from their web servers can come to your computer because you initiated the communication. On the other hand if a malicious website wants to initiate the transfer of data from their website to your computer without your request, your firewall can be programmed to prevent this action based on the fact that you did not first initiate a connection to the website.

Firewalls, if configured properly, can perform many specific tasks to protect the network and computers.