An Amazon Machine Image (AMI) is an encrypted machine image
stored in Amazon S3. It contains all the information
necessary to boot instances of your software.
Once an AMI has been launched, the resulting
running system is referred to as an instance. All
instances based on the same AMI start out
identical and any information on them is lost when
the instances are terminated or fail.
Also known as a security group, groups define firewall rules
that can be shared among a group of instances that have
similar security requirements. The group is specified at
instance launch.