A firewall is a system designed to prevent access from an external network (such as the internet) to an inside private network. The firewall is placed between the untrusted network and the trusted network and monitors all traffic between the two.  Early firewalls tended to just block all inbound traffic and let everything from the inside pass.  This was ok for a while but as threats became more complex, more protection was needed.  There are several different types of firewalls and a short list is provided below:

Packet Firewalls -  All network data travels via packets.  This is the basic transport layer and these firewalls just looked at the packet destination and either allowed or blocked.  A very simple firewall with limited abilities.

Proxy Server – A proxy firewall pretty much intercepts all packets and looks at a rule base to decide its fate.  Proxy firewall included some of the first firewalls but became very cumbersome in functionality.  Although the Proxy firewall is coming back in some respects and is mainly used in conjunction with other firewall types.

Stateful packet filters – This technology checks the state of firewalls connections to determine if the traffic will be allowed through.  An example, is that when the firewall receives a request to allow traffic through such as web traffic it first looks at its connection table to see if there was actually a web request initiated from the inside of the network.  A person browsing the internet for example from the inside network.

Application layer firewall – Application layer firewalls look at the actual information in the packets and at the application layer before allowing a connection.  These types of firewalls are the mainstay of more up to date blended threat firewalls.

This list is not a complete list of the firewall types, but it covers the majority of firewall types.  In the real world, threats have so evolved that the above type firewalls are utilized together to provide the best protection available in a single firewall.  Combining these firewall technologies has helped hinder the latest threats.  We call this Unified Threat Management firewalls.

Firewalls have become a common word in the business world as well as the residential environment.  With the introduction of the internet to business operations, security has become a major concern.  Over the last few years breaches have become a common place and the firewall has evolved.  Over the last few years, the security threats to organizations have dramatically increased.   Along with that threat so has grown the evolution of the firewall.  The name firewall came from the aviation and construction industry.  In the building of an aircraft, a fire protectant firewall was built between the cockpit and the engine compartment on single engine aircraft. This barrier was designed to protect the pilot from the heat and dangers associated with the engine compartment. The construction industry termed this as well when fireproofing buildings with a fire resistant wall in a building.   So came the name firewall as it protects the inside network from the hostile outside environment.