How does firewalls protecting us?

Question

We might not see it, but it's always there and protecting us. But how can they do that?

Answer 1

There are different types of firewalls. Their basic function is to monitor what comes in and goes out of the system. Firewalls examine each packet and blocks the one which doesn't meet the requirement of system and can be harmful for the system. It works upon certain rules as designed by the user.

Answer 2

firewalls work like a filter between your computer/network and the Internet. You can program what you want to get out and what you want to get in. These methods work at different layers of a network.
Firewalls, Only 60 Per Cent Effective Against Malware. ...

Answer 3

A firewall filters and allows traffic in and out by rules like specific ports TCP and/or UDP. The host based firewall will only protect the machine that the firewall is installed on. A network based firewall is there to protect a whole network from malicious traffic from outside (and inside!)
The network admin decides what traffic to allow and what to block. He configures it on the firewall.
Always block all inbound traffic on the firewall by default, unless you have some services running which have to be reachable by the internet.