Hi lynn,

As per ur request, sessions are nothing but the physical
connection establised to oracle database. If u execute
a long running updates or deletes against a table, the session will be
active, this signifies that the table is locked during that
process, simultaneously if another user issue a DML statements
against the same table, his sessions also will be in active mode,
and also it will be realsed only when former job is completed.
this is the overall information, which provided. Default
there are some oracle background process will be running as
soon as the database instance started , theses session information
will active thr' out for ever,until u shutdown the database.