Hey Lawrence,
When you deploy your code to any Cloud it is run on virtual machines (VMs). It is the capabilities of these optimized VMs, and how they can scale to handle any traffic load to your app that makes
App Engine stand out.
App Engine Standard uses lightweight pre-built VMs that run Python 2.7, Java 7, PHP 5.5, and Go 1.6 runtimes. App Engine Flexible on the other hand allows you to create your own runtime using Docker so that you may deploy an application coded in
any language to App Engine.
You can find out more information about the differences between App Engine Standard and Flexible by selecting a language on the
App Engine page.
Note: if you are instead referring to the older App Engine 'Managed VMs', this was indeed updated to become App Engine 'Flexible' last year.