How to have different URLs for microsites?
This is what the Domain Access module is for.  Essentially, it allows you to run multiple Drupal sites out of the same database, and share content.

Sending particular pieces of content to different domains is rather easy; a new tab shows up on edit panels.

It integrates nicely with Panels, Views, and other modules that use cTools, as domain rules show up as selection criteria.

For the Apache side, you need to configure it so that your multiple domains point to the same DOCROOT.  How this is done depends on how you are running your sites.  If you are using virtual hosts, then you set up additional ServerAlias settings for your &lt;VirtualHost&gt;