Block Device Mappings

The advantage of launching an instance from an AMI is that besides mapping to EBS's, the OS and subnet are also mapped. By using AMI, you can easily create the same server on EC2.

For more details, please check here. https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AMIs.html

You can also use shared AMIs or https://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/sharing-amis.html There are also AMIs available for a fee. https://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/paid-amis.html

BlockDeviceMappings, can be a property of the AMI. Examples are definition for the boot volume. You can also specify BlockDeviceMappings in the run-instances api call. Let's say you want to launch a ec2 instance with a multiple EBS volumes. You can specify this BlockDeviceMappings in the run-instances call rather than mapped in the AMI itself.