What is a fire wall?

Java

A system that blocks unwanted connections/communication based on certain filters.

Basically, it can deny NAT redirects, multicast, Java Applets, cookies, PPtp, PPoE, etc.

Also, blocking access to certain web pages by domain name/ keyWords is possible.

Let's say you tried to access runescape

The firewall is set up to block any URL with "runescape" in it.

So the firewall will display a page in your browser saying "access denied" or "forbidden."

So yeah. A firewall can block communications based on their content.

Works for both incoming and outgoing.