a firewall is a deivce that filters all traffic between a protected or "inside" network and a less trustworthy or "outside" network. Usually a firewall runs on a dedicated device;because it is a single point through which traffic is channeled,performance is improtant, which means nonfirewall functions should not be done on the same machine. Because a firewall is executable code, an attacker could compromise that code and execute from the firewall's device .Thus, the fewer pieces of code on the device,the fewer tools the attacker would have by compromising the firewall. Firewall code usually runs on a proprietary or carefully minimized operating system.
The purpose of a firewall is to keep "bad" things outside a protected environment. To accomplish that, firewalls implement a security policy that is specifically designed to address what bad things might happen. For example,the policy might be to prevent any access from outside(while still allowing traffic to pass from the inside to the outside). Alternatively,the policy might permit accesses only from certain places, from certain users, or for certain activities.Part of the challenge of protecting a network with a firewall is detemining which security policy meets the needs of the installation.

这是对防火墙的介绍啊！翻译就不行了，只能看懂！