Session Management - Oracle 10g/11g Data and Database Management Utilities - page 186

Here you can see the time frame specification and the report header, for your
convenience the report can be saved in HTML format for further analysis.
The Active Session History report is produced out of several AWR tables, the AWR
takes a periodic activity snapshot and it stores the information for a given period of
time, seven days by default. The DBA views that can be used to read the historical
information from the AWR are listed with this query.
SELECT table_name
FROM dictionary
WHERE table_name like 'DBA/_HIST/_%' ESCAPE '/'
ORDER BY table_name;
The V$ACTIVE_SESSION_HISTORY view provides sampled session activity in the
instance. Active sessions are sampled every second and are stored in a circular buffer
in SGA. An active session is defined as a session connected to the database that is
waiting for an event that does not belong to the Idle wait class.