A firewall is a network security device that monitors and controls incoming and outgoing traffic from a computer network. It allows or blocks traffic based on a defined set of security rules. By allowing only authorized traffic and blocking unwanted traffic, firewalls protect against unauthorized access, malware, and other security threats.

Firewalls can be delivered as a hardware appliance, as software, or software as a service (SaaS). They can be divided into two general types: network firewalls and host-based firewalls. A host-based or computer firewall protects just one computer, or "host," and are typically deployed on home or personal devices. Network firewalls, on the other hand, protect all devices and traffic passing a demarcation point, enabling broad scalability.

There are several types of firewalls depending on their operational method:

1. Stateless or packet filtering firewall
2. Stateful inspection firewall
3. Circuit-level gateway
4. Application-level gateway
5. Next-generation firewall (NGFW)

Each type of firewall has its own strengths and is used based on the specific needs of the network. It's important to note that while both firewalls and antivirus software protect you from threats, the ways they go about doing so are different. A firewall filters traffic that enters and exits your network, while antivirus software works by scanning devices and storage systems on your network looking for threats that have already penetrated your defenses.