The best thing will be if you can get hold of "Top Sessions" tool of Oracle Enterprise Manager. This has a very good GUI that shows you detailed information about the current sessions and their activities.
It actually uses the following objects to gather the info --

V$SESSION
V$STATNAME
V$SESSTAT
ALL_TAB_COLUMNS
V$OPEN_CURSOR
V$SQLTEXT
V$LOCK
SYS.DBA_OBJECTS
V$ROLLNAME
PLAN_TABLE
ALL_CATALOG
AUDIT_ACTIONS

Also make sure that you didn't create any Management Server's repository on this database. MAnagement server pings the database at a set interval to check the status of the services you are monitoring (this causes transaction information).

- Rajeev