Suppose you want to graph average salaries by department, for all employees.
Your application needs to know whenever EMP is changed.
Your application would look similar to this code: 
DBMS_ALERT.REGISTER('emp_table_alert');
    readagain: 
    /* ... read the emp table and graph it */  
      DBMS_ALERT.WAITONE('emp_table_alert', :message, :status); 
      if status =  0  then goto readagain; else 
       /* ... error condition */  

The EMP table would have a trigger similar to this: 
CREATE TRIGGER emptrig AFTER INSERT OR UPDATE OR DELETE ON emp
    BEGIN 
      DBMS_ALERT.SIGNAL('emp_table_alert', 'message_text'); 
   END;

When the application is no longer interested in the alert, it makes this request: 
DBMS_ALERT.REMOVE('emp_table_alert');

Syntax
DBMS_ALERT.WAITONE (
   name      IN   VARCHAR2,
   message   OUT  VARCHAR2,
   status    OUT  INTEGER,
   timeout   IN   NUMBER DEFAULT MAXWAIT);