So I have my host up and running.
billybob.com

I have multiple subdomains up and running
dev.billybob.com

Apache is happy serving different sites to these domains.

I have created and setup another site for a different project and registered a new domain
joeblogs.com
and I have pointed that site to the linode name servers.

I want to host on the same server.
In the linode dns manager do I need to create a new zone file for joeblogs.com or do I just add an a record to the zone file for billybob.com similar to were the dev subdomain entry is?

I can't seem to find a clear example.

del

worked it out. New zone file I think. Well it works.

Edit: kind of

.www.joeblogs.com works

but

joeblogs.com is going to my default apache setup.
.

Looks like its time to delve into the apache side of the config.

You probably need to add joeblogs.com to the ServerAlias directive in the appropriate config file. On Ubuntu/Debian, this can be found under /etc/apache2/sites-available. Don't forget to tell Apache to reload its config.