Explain :NEW and :OLD in row-level triggers

In a FOR EACH ROW trigger, :OLD and :NEW are pseudorecords shaped like the table. On INSERT, :OLD is NULL and :NEW is the incoming row. On UPDATE both are set. On DELETE, :NEW is NULL and :OLD is the row being removed. In a BEFORE row trigger you may assign to :NEW.column to change what gets written (e.g. default or normalise a value); in AFTER triggers :NEW is read-only. In the WHEN clause and SQL statements, drop the colon (NEW.salary).

CREATE OR REPLACE TRIGGER trg_emp_normalise
  BEFORE INSERT OR UPDATE ON employees
  FOR EACH ROW
BEGIN
  :NEW.email := UPPER(TRIM(:NEW.email));
  IF INSERTING THEN
    :NEW.created_on := SYSTIMESTAMP;
  END IF;
  :NEW.updated_on := SYSTIMESTAMP;
END;
/