Firewall

Here you'll find all about it:

http://www.pc-help.org/www.nwinternet.com/pchelp/security/firewalls.htm

use this site and you find geart information

sans.org

Firewall, also known as a Bastion Host, is normally a multi-homed (more than one NIC) system running some form of packet inspection software.

Firewalls can be simply software, like Checkpoint, or MS ISA Server that you install on a multi-homed system, or it can be a hardware solution like the Cisco PIX or Sidewinder, which is a multi-homed hardware appliance that is configured with it's own vendor proprietary IOS.

Additionally there are various levels of inspection performed, everything from simple NAT boxes and Access List inspection (think of access lists like a doorman at a bar, if your name is not on the list, or if you did not leave out the door he is watching, then you are not coming in through that door), to Stateful Packet Inspection which examines more closely at the "State" of the packet in relation to the established or establishing session, and Application level firewalls which actually look at the nature of the actions and requests occuring in the session.

There are even application specific Firewalls such as eEye Digital Security's "Secure IIS", which is designed to protect only Microsoft Web Servers.

Hope this helped

Chris Weber CCDP