A firewall is a software or hardware device used to prevent people from getting into your computer by blocking intrusion attempts from harmful hackers (or crackers).
The best firewall is offered in a hardware device called a "router". These devices provide superior protection against outside computers trying to access your computer. Routers also have the ability to connect extra computers to your Internet connection, and to each other.
Hackers have hundreds of tools for breaking into computers. Some of the symptoms of someone else utilizing your computer:
Update to the latest security packs from Microsoft and Apple.
For more information on routers, you should want to contact your local computer dealer. Some common brands are:
For software firewalls on your computer: