Google has announced machine images, a new type of Compute Engine resource that contains all the information people need to create, backup or restore a virtual machine, reducing the amount of time people spend managing their Cloud environment.
Custom images capture the contents of a single disk, for example, a boot disk, which can be used to create new instances that are preconfigured with the apps that you need.
Machine images are a more comprehensive resource that can contain multiple disks, as well as all of the information required to capture and create a new instance.