Question on urls
 

I'm setting up a public *Nix server that will host several games, including Nethack.
My question I cannot easily put into words, simply it is,
What is this portion of a URL for?
-->(nethack.)alt.org
(Of course there wouldn't really be parentheses or an arrow :P)

This will probably be an A record which identifies a specific hostname to IP address mapping, it could also be a CNAME record which would be an alias to another host.

Fake A Record: www.domain.com 127.0.0.1
Fake CNAME Record: games.domain.com -> www.domain.com

<host_or_alias>.<domain>.<top_level_domain>