On the cloud you can run your site using either a Windows, Linux or hybrid backend, which you should use depends on your circumstances. If in doubt, the Linux platform will usually be the one you want to use.

Linux: Linux is the recommended backend if you are planning to use PHP based software. Common examples include Wordpress, Magento, Joomla, etc. By using linux you have control of both the PHP version as well as the ability to manage PHP settings through the use of a custom php.ini file. 

Windows: Windows is the platform you will need to use if you wish to run ASP/.NET based software, an example of which is Umbraco. You can run PHP through Windows, but it is much slower and less customisable due to how PHP functions within a Windows environment.

Hybrid: Hybrid mode will run your PHP code through our linux cluster and your ASP through our Windows cluster. This means you can have PHP and ASP code running within the same sites. The bias determines whether code other than PHP/ASP (ie raw HTML) through Windows or Linux.

N.B!: If you use Windows on your home computer, this does not mean that you need to run your site under Windows on the cloud as it refers to the backend system rather than the one you're using to access/modify your site.