Amazon Instance Stores

EC2 Storage Options: Instance Store, EBS Backed and Snapshots

Storage for the root device of an EC2 instance can be either Instance Store (ephemeral storage) or EBS backed volumes. Ephemeral disk is a temporary storage that it is added to your instance and is not persistant. An Amazon EBS volume is a durable, block-level storage device that you can attach to a single EC2 instance. 

Choice of AMIs depends also on root device types. Root device type is displayed on the AMI selection page.