An Amazon Machine Image (AMI) is a master image for the creation of virtual servers -- known as EC2 instances -- in the Amazon Web Services (AWS) environment.
The machine images are like templates that are configured with an operating system and other software that determine the user's operating environment. You must specify an AMI when you launch an instance. You can launch multiple instances from a single AMI when you require multiple instances with the same configuration.
AMIs are region specific. An AMI created for US-East-1 cannot be used to create an EC2 in US-West-1 for example.
You can select an AMI to use based on the following characteristics:
Add your own software, configuration, operation system, monitoring...
Faster Boot/configuration time because all your software is pre-packaged
AMI Built for specific regions (Can be copied across regions)
you can launch EC2 instances from: