A CRT is based upon the principle that when electrons hit phosphorous, the phosphorous glows, like in a neon bulb. A CRT is cone-shaped like a funnel, with a slightly rounded glass plate at the large end. This is the side placed towards the viewer. The glass plate is coated on the inside with phosphorous. At the small end of the funnel, a device called an electron gun is placed. The gun shoots out a stream of electrons, which hits the large glass plate. When the beam hits the phosphorous coating, the target spot glows.
Having a gun shoot an electron beam is not enough. Without beam movement, all you would see is just one little speck of light on the screen. In order for you to see a picture, the beam has to sweep rapidly from side to side and up and down. In order to do this, the beam, after leaving the gun, is "warped", or pulled by magnetic fields. To demonstrate this, roll a steel marble on a table. Notice that it rolls in a straight line. Next, place a magnet on the table. Roll the marble past the magnet. Notice how the magnetic field alters the course of the marble. Vertical and horizontal deflection plates are like the magnet. They pull electrons to one side or the other, depending on the magnetic charge placed on the plate.
A CRT has four deflection plates. Two plates control the horizontal, or side to side deflection of the beam, These are the horizontal control plates. Two plates control the vertical, or up and down, deflection of the beam. These are the vertical deflection plates. Think of this as a box, with the beam shooting through the box. The beam itself is deflected thousands of times per second, depending on the varying charge on the plates. The magnetic charge itself is controlled by various electronic components within the television housing.
You now understand how a CRT works, and what deflection plates are. The deflection plates themselves are placed opposite of the beam movement. The horizontal deflection plates are vertical, like the walls in your house. The vertical deflection plates are placed horizontally, like the floor and the ceiling. The lower vertical deflection plate is like the floor. It pulls the electron beam down towards the bottom of the screen when activated.