The Automatic Workload Repository
Oracle collect a vast amount of statistics regarding the performance and activity. These statistics is accumulated in the memory and periodically flushed into AWR. The AWR is a set of tables and other data structure in the SYSAUX tablespace. By default the statistics are flushed into AWR once a hour, this is known as the AWR snapshot. The AWR snapshot will be stored in the AWR for 8 days by default. The collection for system statistics have three level-basic,typical and all. The default is typical. Basic gather too less statistics which may be helpless. All will gather too much statistics which may affect the database performance. To set the level you can specify the value of initialization parameter STATISTICS_LEVEL.
The content of AWR snapshot can be seen as the collection of all the performance v$ views. In addation to this, the AWR snapshot can also store information from DBA views. For example, the history of a data object statistics. With the AWR, database will not have a long-term record of how data object were changing. The DBMS_STATS will return current statistics but a historical picture of the state of the data objects may also very useful.
You can use the AWR snapshot to generate AWR report.
The ADDM
The database comes with a set of preconfigured advisors. We can say Automatic Database Diagnostic Monitor is the interface for this advisors. ADDM will generat a ADDM report everytime a AWR snapshot generated. The ADDM will point some issues the database may facing and will suggest you to do something. The something here may means run other advisors.
The ADDM is run automatically by the MMON whenever a snapshot is taken. As with all the advisors, it takes statistics and other information from the AWR. The automatically generated ADDM reports always cover the period between the current snapshot and the previous one—so by default, you will have access to reports covering every hour. You can invoke the ADDM manually to generate a report covering the period between any two snapshots, if you want to span a greater period. The ADDM report will be purged 30 days.