What are firewalls intended to accomplish?

Answer 1

Firewalls protect one computer (or network) from another computer (or network) by imposing limits on how those two computers (or networks) can interact.

In one example, you might have several computers in your internal network. You can control them by using telnet. You could use a firewall between your internal network and an external network to prevent a computer in the external network from accessing a computer in the internal network using telnet. Since telnet can be a security risk, blocking external telnet like this is highly appropriate.

In another example, you might have a private firewall on your computer. You use the computer to browse other computers using http. You firewall could be configured to ensure that only the processes iexplore.exe or firefox.exe were allowed to use http. This way, if some malicious program, for instance malware.exe, attempted to use http, it would be blocked. Blocking can also be applied to external sites by address.

There are many possibilities... Most modern firewalls are extremely complex, because there are many different possible interactions. Some are good, and some are bad.