The Domain Mapper add-on lets you use one concrete5 install to serve content at multiple domains. You are still effectively running one website in the sense that there will be shared users, groups, files, themes, attributes, add-ons, etc. You can even copy pages from one "site" to another easily with this technique. What the Domain Mapper lets you do is point parked domains to specific spots on your site tree as a starting point for that domain. An example:
Turn full page caching off and try again.
Full page caching was introduced in concrete5 version 5.4.1. The point of full page caching is to display a cached page using very little resources. The domain mapper ties into one of the events that is skipped durring the rendering of a cached page so it's logic is never run.