Amazon CloudWatch collects raw data from partnered AWS services such as Amazon EC2 and then processes the information into readable, near real-time metrics. These statistics are recorded for a period of two weeks, allowing you access to historical information and providing you with a better perspective on how your web application or service is performing.
Amazon CloudWatch runs a monitoring services that collects raw measurement data or measures, such as CPUUtilization (percentage of Amazon EC2 compute units used by an instance) or DiskWriteBytes (number of bytes written in a minute). Measures consist of a name (e.g., DiskWriteBytes), a value (e.g., 0 bytes per second), additional metadata that provides more context, and a timestamp.
For more information, refer to the Amazon CloudWatch Developer Guide.