AWS services send metrics automatically for free to CloudWatch at a rate of one data point per metric per 5-minute interval, without you needing to do anything to turn on that data collection.
Get more granularity with extra fee associated by posting metrics every minute instead of every 5 minutes.
Each metric in CloudWatch has a timestamp and is organized into containers called namespaces. Metrics in different namespaces are isolated from each other.
AWS services that send data to CloudWatch attach dimensions to each metric. A dimension is a name/value pair that is part of the metric’s identity. You can use dimensions to filter the results. For example, you can get statistics for a specific EC2 instance by specifying the InstanceId dimension when you search.
Custom metrics allows you to publish your own metrics to CloudWatch.