According to the online documentation:
Refresh – Repaints the control on the screen.
Call Refresh method to repaint the
control immediately. Refresh calls the
Repaint method. Use the Refresh and
Repaint methods interchangeably.
Repaint – Forces the control to repaint its image on the screen.
Call Repaint to force the control to
repaint its image immediately. If the
ControlStyle property includes
csOpaque, the control paints itself
directly. Otherwise, the Repaint
method calls the Invalidate method and
then the Update method so that any
visible portions of controls beneath
the control will be repainted as well.
Update – Processes any pending paint messages immediately.
Call Update to force the control to be
repainted before any more, possibly
time-consuming, processing takes
place. Use Update to provide immediate
feedback to the user that cannot wait
for the Windows paint message to
arrive.Update does not invalidate the
control, but simply forces a repaint
of any regions that have already been
invalidated. Call Repaint instead to
invalidate the control as well.