Virtual Machine in Google Cloud Platform
Virtual Machine is a software computer that, like a physical computer, it runs an operating system and applications. The virtual machine is comprised of a set of specification and configuration files and is backed by the physical resources of a host.
Google App Engine
Google App Engine applications are easy to create and easy to maintain. It easily scales your applications as your traffic and data storage needs change. Their are no servers to maintain in App Engine. All you need to do is to simply upload your application and it's ready to go.
Migrating From VMs to Compute Engine
While you are moving to cloud, it is very important to consider tiny things which can makes your whole journey of things going worse. Migration to cloud is a simple thing having few minor steps to follow. But you should know the pros and cons of that.