Amazon Machine Image (AMI) is a template in which we can store the configuration of a server. It can be an Operating System, Application Server, and Web Server etc.
An Amazon Instance is the actual server that is built by using an AMI. Amazon Instance typically runs in Amazon AWS cloud.
We can even launch multiple types of instances from the same AMI. This is possible by passing arguments to an AMI. Every Instance can have different computing and memory configuration.Read the full book at www.amazon.com