What ever the type of firewall in any Linux systems, they receive and then inspect and make a decisions upon all the net traffic that goes through in to perimeters. basic some type of firewall does packet filtering. Through this type of filtering all internet data is broken up into small parts or packets and then monitored. If it is not registered in a virus, That firewall prevents the data from entering your computer. The second type of firewall system is circuit relay or circuit level gateway. Here the firewall checks the authenticity of connections before allowing data traffic. It permits traffic from a permitted sources for a limited period  time. In the advanced firewall security system, the application level gateway is considered to be a most secure of all in computer. It acts as a proxy and devoted detailed attention to any application that uses the gateway.

Linux Firewall:-

A Linux is the secure enough to run on your computer provided you just work on it as a client, But once it is connected to remote services, you need to make use a firewalls. A firewall is the first step you take in an elaborately planned security system. There are two kinds of user accounts. One is a low privileged user account and the other one  is a full privileged user account. Most Linux distributors set up low privileged user accounts in their computers. If yours is low privileged it prevents malicious viruses from it attacking your computer. In extreme cases, some viruses or hackers might try to damage your files & data. This can be prevented by logging in through the root accounts. Once you are logged in through the root account it is powerful enough that you can change your Linux configurations. This places Linux users in an advantageous position because such a facility is not available with Windows users as they are logged in full time as a full privileged user. But to be on the safe side please keep on updating the firewall regularly. You also have to make sure that no one logs in as a root user and uses unsecured sites enabling viruses to attack your computer. If this happens viruses which are known to exist in the Linux systems can come in and steal user data.Be safe