apache 2.2 - Difference between ServerName and ServerAlias

I'm not quite clear on the difference between ServerName and ServerAlias. It looks like both of them work as host name settings, except that ServerAlias only works within the tag.

That is, I can do:

ServerName www.domain1.com
ServerName www.domain2.com

or:

ServerName www.domain1.com
ServerName www.domain2.com

and both domains work on the same box. Can I use either ServerName or ServerAlias in this case?

Answer

The ServerName directive is

Hostname and port that the server uses to identify itself

Whilst ServerAlias is

Alternate names for a host used when matching requests to name-virtual hosts

Given a vhost configured like

 ...
 ServerName example.com
 ServerAlias www.example.com foo.example.com *.somewherelse.org
 ...

apache would respond to example.com, www.example.com foo.example.com and anything in .somewherelse.org with this VirtualHost