This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
A firewall is a program/server that's placed between two servers to limit access. The following URL talks about firewalls from the world wide web point of view. But you can implement firewalls for other reasons. Let's say you are a wholesaler and have a server through which you allow your customers to gain access to your order entry/status checking application. On that server you might only have software to verify the identity of user and launch the actual application over the firewall. So the actuall application is on a server internal to the company (the other side of the firewall) and is being implemented on the public side of the firewall. This limits the customer or hackers from messing around with your application. Of course, some hackers can break through the firewall. Then you put up more defense - bastion hosts. You may learn more at the following url. http://www.greatcircle.com/gca/tutorial/bif.html#firewall