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.
A set of customer instances that have been designated by the customer as being related by assigning them the same security group when the instances were first run. The Amazon EC2 firewall controls access to instances based on the instance's group membership and the rules defined for the group.
A collection if instances started as part of the same launch request.