An Amazon Machine Image (AMI) provides the information required to launch an EC2 instance.
You specify an AMI when you launch an instance, and you can launch as many instances from the AMI as you need.
You can also launch instances from as many different AMIs as you need.
An AMI includes the following:
A template for the root volume for the instance (for example, an operating system, an application server, and applications)
Launch permissions that control which AWS accounts can use the AMI to launch instances
A block device mapping that specifies the volumes to attach to the instance when it's launched
AMIs are regional. Can launch an AMI only from the region in which it is stored. Can however store AMI to other regions using console, command line or the EC2 API.
Public AMI Publishing: Hardening and Clean-up Requirements: https://aws.amazon.com/articles/9001172542712674
Guidelines for Shared Linux AMIs: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/building-shared-amis.html