Firewalls are used to mediate and control all information
that is communicated between an external (untrusted) network
and an
internal (trusted) network. Firewalls make use of IP
filtering and application proxies to implement firewall
security policies.

The foremost purpose of Firewall is to prevent the 
unauthenticated access of several resources of your network 
as well as protection from ID-Theft scams such as spoofing 
and phishing