Auto Scaling Groups, or ASGs, are a way to automate the expansion or shrinking of your number of EC2 instances based on usage. You set a minimum number of instances, a maximum number of instances, and your desired number and the ASG does the scaling as needed. You are only charged for the instances you use, the ASG is free. You first start with a launch configuration, detailing the AMI, network, etc. Then you can even attach the ASG to an ELB meaning that you can have the ASG attach instances to your load balancer right away with no need to configure. If an instance is unhealthy the ASG will remove and replace it and you can even set it to where your ASG will also allow tests from the ELB to determine if an instance is failed and replace in those cases as well. The ASG is a great way to only pay for what you use.