what's a valid domain name?

[pyh at mail.nsbeta.info]

Mark Andrews writes:
> 
> Remember domain names are not the same things as hostnames.  Hostnames
> are limited to letter digit hyphen (LDH).  Other domain names are not
> limited and things last SRV records deliberately use labels that are
> not legal in hostnames to prevent collisions of the two namespaces when
> they are stored in the DNS. 
> 

Could you show an example for the difference between a hostname and a 
domain name? 

Thanks again. 

Regards.