Automatic Performance Statistics

http://docs.oracle.com/cd/E11882_01/server.112/e16638/autostat.htm

• When analyzing a performance problem in any of these scopes, you typically look at the change in statistics (delta value) over the period you are interested in. Specifically, you look at the difference between the cumulative value of a statistic at the start of the period and the cumulative value at the end.
• Cumulative values for statistics are generally available through dynamic performance views, such as the V$SESSTAT and V$SYSSTAT views
• Note that the cumulative values in dynamic views are reset when the database instance is shutdown
• The Automatic Workload Repository (AWR) automatically persists the cumulative and delta values for most of the statistics at all levels except the session level. 
• This process is repeated on a regular time period and the result is called an AWR snapshot. The delta values captured by the snapshot represent the changes for each statistic over the time period