Using Amazon CloudWatch Metrics

Metrics are data about the performance of your systems. By default, several services
provide free metrics for resources (such as Amazon EC2 instances, Amazon EBS volumes,
and Amazon RDS DB
instances). You can also enable detailed monitoring some resources, such as your Amazon
EC2
instances, or publish your own application metrics. Amazon CloudWatch can load all
the metrics
in your account (both AWS resource metrics and application metrics that you provide)
for search, graphing, and alarms.

Metric data is kept for a period of 15 months, enabling you to view both up-to-the-minute
data and historical data.