A FIREWALL is a part of a system or a network which has as its purpose to lock the access of unauthorized users and to permit the access of authorized ones. It is a device or a set of devices configured to permit, deny, encrypt or decrypt the data traffic based on a set of rules and other criteria.
The firewalls can be hardware or software, or a combination of them. They are frequently used to prevent the access of unauthorized users to private networks connected to the internet. So thus the entering or outgoing data traffic crosses the firewall which examines each message and locks the ones that don’t fit the set of rules and criteria specified by rules of safety.