Error - "Sorry, apps are turned off. If you know who runs the server, tell them to enable apps."

you will see this error when you try to add SharePoint Hosted app :

Sorry, apps are turned off. If you know who runs the server, tell them to enable apps.

Solution:

In order to install an App from the App Store you must setup an isolated App Domain, this is because Apps from the store deploy to their own app domain and run under a separate process from your SharePoint sites.

If your SharePoint site uses the DNS alias http://sharepoint.contoso.com you could have *.apps.contoso.com as your app domain.

http://www.sharepointalex.co.uk/index.php/2012/11/enabling-sharepoint-2013-apps/

How to: Set up an on-premises development environment for apps for SharePoint

http://msdn.microsoft.com/en-us/library/fp179923(v=office.15).aspx