Horizonal shadow lines on Monitor

Why does the monitor have a horizontal shadow on types lines or lines of Icons. They can be seen faintly on the desktop also. They are most predominant when typing in Word or just displaying the “Control Panel” or “My Computer”. On either of these screens, the Icons and Title Text under have the horizontal shadows. All of the space between a line of text and the next row of Icons is a crisp white as it should be.