Amazon CloudWatch is a monitoring service for AWS cloud resources and the applications you run onAWS.
You can use Amazon CloudWatch to collect and track metrics, collect and monitor log files, and setalarms.
When you create an alarm, you first choose the Amazon CloudWatch metric you want it to monitor.Next, you choose the evaluation period (e.g., five minutes or one hour) and a statistical value tomeasure (e.g., Average or Maximum).
To set a threshold, set a target value and choose whether the alarm will trigger when the value isgreater than (>), greater than or equal to (>=), less than (<), or less than or equal to (<=) that value.
Reference:
http://aws.amazon.com/cloudwatch/faqs/