Firewalls are tools that are used to enhance the security of computers connected to a network, such as a Local Area Network or the Internet. Firewalls deny or accept all messages and sites based on a list that is stored in the system. This is usually decided by a system administrator.
The two kinds of firewall mechanisms are namely packet filtering and stateful packet inspection.
The former does not check the content of a message where as the later checks the content, thereby providing more security.