You can collect basic metrics such as CPU utilization, network I/O, and disk I/O metrics from your EC2 instances. There are two types of CloudWatch monitoring services—basic and detailed. In basic monitoring, the Amazon EC2 metrics data is collected at 5 minute periods and retained for 2 weeks, at no charge. The metrics are preselected and limited in number. Basic monitoring is automatically enabled for all EC2 instances. In detailed monitoring, the metrics are collected at 1-minute intervals and charged per hour per instance.
Regardless of the monitoring type, the metrics are aggregated by autoscaling group and EBS (if you are using autoscaling or EBS). If you want detailed monitoring, you can enable it, both while creating a new EC2 instance and for existing EC2 instances. You can access the metrics data using either the CloudWatch API or the AWS Management Console.