Apache 101 help !

Hi,

I am new to the web server .... please help me with this 2 questions.


1/ Setup the basic web server myhost.mydomain.com
2/ Setup the virtual host www.mydomain.com


My linux system hostname is: myhost.mydomain.com
IP: 1.2.3.4
DNS: www.mydomain.com CNAME myhost.mydomain.com


Please explain this:
If I use default DocumentRoot /var/www/html for question #1, and use
VirtualHost for #2, I no longer see the index.html file from
DocumentRoot of #1


Question #1 does not asked to use the Virtual host.
Can I just use the default DocumentRoot to config the web server OR
have to use the Virtual host ?


Thanks
Tam