Web hosting security?

[geo]

How does this work then?

On my computer the web directory is /var/www/

On the internet someone types in http://something.whatever or an ip address.

What prevents a person on the internet (read: Hacker) from crawling up my computer's directory tree from /var/www/ to /var/ to /

Can anyone explain this?

Do I need to do anything to make the computer defended? I am behind a firewall (D-Link) and only have port 80 open.

Do I need a software firewall like Firestarter? Or will Apache take care of itself?

How does this work?

Thanks,
geo