A firewall sits between your computer and the rest of the Internet, filtering out unwanted traffic and foiling attempts to interfere with or take over your computer. Firewalls can be separate devices, which is very common today, or simply pieces of software for your own computer, which is also fairly common. Separate firewall devices are often preferable because their very simplicity makes them less likely to have unknown security problems; however, it is still important to keep up with "firmware updates" released by the manufacturer, otherwise your firewall may be vulnerable to attack. "Cable/DSL routers" and similar devices sold by companies like Linksys provide simple firewall capabilities which are adequate for most home users.
