Firewall is a device or a component that restricts access between a protected or an internal network from an external or untrustworthy network .A firewall basically limits unauthorized network access. It acts as a guard between an internal private network or intranet and an external public network or the Internet. As it is present at a junction of two networks it allows only a selective type of traffic to pass through. In this way a firewall provides a way to implement security policies for more secure networks .

Firewalls Uses
The main use of firewalls is for security purposes. Each company devise some security policies to make its private networks secure. But to put these policies into action only firewalls provide reasonable solutions.
Firewalls can help an enterprise to protect its confidential information from being stolen and stop intruders from entering their secure networks. They can also generate useful information by monitoring internal communications and can generate alarms in case of information breach. Common technical threats faced by companies include:

IP Spoofing: A technique used to gain unauthorized computer access by disguising that an IP address is coming from a trusted source.

ICMP Bombing: The Internet Control Message Protocol (ICMP) is used by the Internet routers to notify a client machine when the specified destination is unreachable. An attacker can bombard a computer with bogus ICMP messages to knock it off the Internet.

FTP Bouncing: It is used to send flood of messages to multiple recipients thus clogging their inboxes also known as email bombs.
Other popular threats include attacks from curious and malicious hackers, spying, accidental disclosure of company data and denial-of-service (DOS) attacks