AWR is the best tool available to identify performance issues on your database. It has several tools to help you identify performance issues and all the information is displayed in a single HTML page. AWR periodically gathers and stores system activity and workload data which is then analyzed by ADDM.

Every layer of Oracle is equipped with instrumentation that gathers information on workload which will then be used to make self-managing decisions. AWR is the place where this data is stored. AWR looks periodically at the system performance (by default every 60 minutes) and stores the information found (by default up to 7 days). AWR runs by default and Oracle states that it does not add a noticeable level of overhead. A new background server process (MMON) takes snapshots of the in-memory database statistics (much like STATSPACK) and stores this information in the repository.

For more details on its usage, please ckick HERE