A cathode ray tube consists of three primary components. An electron gun, two pairs of deflecting magnents and a phosphor-coated screen. (For the sake of simplicity, the model described here is greatly simplified.) The electron gun produces a continuous beam of electrons. This beam is emitted in the direction of the screen between the two pairs of deflecting magnets. By varying the strength of the magnetic field produced by these magnets, the beam can be deflected to strike the desired portion of the screen. One pair of magnets control deflection in the horizontal direction, the other pair controls deflection in the vertical direction. When the beam strikes the screen, the phosphor flouresces and emits light whose internsity is proportional to the strength and duration of the electron beam.
If the beam shines continuously on a single point, a white dot is seen. If the beam is moved, a curve of gradually fading intensity is seen. The beam can immediately retrace this curve and reilluminate the curve. The number of times per second that the beam illuminates a particular point is know as the refresh rate for the CRT. Refresh rates of roughly greater than 20-30 produce images in which the tracing is not visible. Instead, a steady intensity is seen. This same phenomena, persistence of vision, is used in motion pictures and animation to simulate continuous motion.