A firewall is a network security device that is either a hardware or software which monitors all the incoming and outgoing traffic. Based on a defined set of security rules it accepts, rejects or drops the specific traffic. The modes are as follows : 

• Accept : Accepts and allows the traffic in the network
• Reject : Blocks the traffic but replies with an unreachable error
• Drop : Blocks the traffic with no reply

Firewall, in other words, serves as the boundary between the internal secured networks and outside untrusted network such as the Internet.

Firewalls are of two types : 

• Host-based firewall : It protects each and every individual nodes from attacks and unauthorized access
• Network-based firewall : It protects the internal network 

Categorization of firewall based on generation :

• First generation - Packet Filtering Firewall
• Second generation : Stateful Inspection Firewall
• Third generation : Application Layer Firewall
• Next Generation Firewalls (NGFW)

In Windows, already a firewall is installed that is Windows Firewall.