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 is added to your instance and is not persistant.
EBS volume is a durable, block-level storage device that you can attach to a single EC2 instance.
Root device type of an AMI is displayed on the AMI selection page.
Instance stores vs. EBS Volumes
Instance stores are disks that are physically attached to the host computer that provide temporary block-level storage for instances.
The data on an instance store volume persists only during the life of the associated instance; if you terminate an instance, any data on instance store volumes is lost.
EBS volumes persist independently from the running life of an EC2 instance.
You can attach additional EBS volumes even after launching an instance.
You can detach an EBS volume from one instance and attach it to another instance in the same Availability Zone..
Instances with instance-stores cannot be stopped, can only be rebooted or terminated. Therefore, you get less durability compared to instances with ebs root device: Stopping and starting an instance is a solution most of the time when a host fails.
While launching an instance, instance store volumes are created from a template stored in S3, whereas, EBS volumes are created from EBS snapshots. This can lead to a slightly more time to launch instance store backed instances compared to EBS backed instances.