An instance snapshot is a copy if an instance (virtual machine) made at a specific moment in time. Instance snapshot allows you to restore that virtual machine back to that specific point in time when necessary, or recreate the machine later in case it was deleted.
Such snapshot is created my making a copy of a virtual machine, and storing this copy in the image repository. Technically, instance snapshots are very similar to images, and they can be used for the same scenarios. In the API and command line tools, you will see these snapshots as images. However, the objects created as snapshots have different (from images) properties, so they are shown in a different section of the portal, and you can easily distinguish images (which purpose is to provide a template for a repeatable instance creation) from snapshots (which are used as backup and recovery mechanism for earlier created VMs)
Note that snapshots are billable resources; see details in the billing section.
To create an instance snapshot, follow these steps:
In order to view your snapshots, navigate to the "Snapshots" page on the left pane of the portal, under the "Images" section. In this page, you can view all created snapshots, and you can filter them by name or select one of the predefined filters to filter by the OS type.
To restore an instance from a snapshot, simply create a new instance from that snapshot - same way as you do when creating instances from an image. To do that, click on the snapshot action button, and select "Launch new instance". Alternatively, you can select the snapshot as source for the instance, during the regular instance creation process.
To delete a snapshot, click on the snapshot action button, and select "Delete". Note that you can't delete a snapshot which has instances deployed from it - you will need to delete those instances first. The original instance (source of the snapshot) can be deleted at any time, before or after the new instance creation.