A firewall is a device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass.
A Firewall is a security system that adds a level of protection between your computer and the internet. Firewall helps to prevent viruses and worms to enter into your computer. If one of the computers is infected, firewall on each computer help to prevent the spread of virus in a network
Many personal computer operating systems include software-based firewalls to protect against threats from the public Internet. Many routers that pass data between networks contain firewall components and, conversely, many firewalls can perform basic routing functions.