What Is a Firewall?
A firewall is a program or hardware device that watches your network and Internet connection and helps block unauthorized access to your computer. The firewall controls what data can come across the thousands of ports that your computer can use to communicate with the outside world.
Hardware firewalls are built into networking devices such as routers and wireless access points. Most consumer hardware firewalls only watch traffic between the network device and the Internet—they do not block traffic between computers inside your network. Software firewalls are programs installed on your PC.
Do I Really Need a Firewall?
Can I Use More than One Software Firewall on My System?
Is Opening Firewall Ports Risky?