Session-related statistics

V$SESSION: stores current session information for each user logged on.
V$SESSION or V$SESSION_WAIT: used to determine the resources or events for which active sessions are waiting.
V$SESSTAT: displays user session statistics.
V$SESSION_EVENT: list information on waits for an event by a session.
V$SESSTAT and V$SYSSTAT: cumulative values for statistics are generally available through these two views.
V$MYSTAT: displays the statistics for the current session.
V$SESSMETRIC: display the performance metric values for all active sessions. The view lists performance metrics such as CPU usage, number of physical reads, number of hard parses, and the logical read ratio.

Oracle data dictionary views
Oracle dynamic performance views