firewall

You may want to visit this site and search for the term "Firewall" ... (without the quotes).

http://whatis.com/

basicly a firewall is the network electronic version of a doorman/guard. It checks everything passing througn the gateway (the doorway between your network and another, usually the Internet) and does not allow it through unless it confirms to the approved format you have set out in the firewall set up; like an exclusive club's doorman excluding someone in a bad t-shirt.

There is a lot of info about types of firewalls and which you chould use and how you set them up etc; but they all stop unapproved traffic, or try to anyway.

There are a number of good links at www.firewall.com

The term firewall comes from the construction industry.
There, a firewall is a specially constructed wall that stops or slows the progression of a fire in a building.

A computer firewall issoftware, or firmware, the monitors the flow of data on a network, allowing only those users who identify themselves, and who are authorized to use the network.

(it's not nearly that simple)

You need a firewall on any connection between your network, and the rest of the world.
The most cited reason, is to keep 'hackers' out.
You also want to keep disgruntled employees from gaining access.
Along with industrial spy's.

Chas