A beam of electrons is shot down an evacuated tube by an electron gun. This beam passes between two sets of deflecting plates and then strikes a screen that glows when the beam hits it.
The most important parts of the CR0 are:
(a) electron gun - this produces the electron beam. The more electrons there are in the beam the brighter the spot on the screen;
(b) deflecting plates - by putting a voltage on each pair of plates the beam can be moved up and down or side to side. The electron beam is negative so it will be attracted towards a positive plate;
(c) fluorescent screen - this glows when the electron beam hits it.
You will find various controls on the front of an oscilloscope, this is what they do:
Brightness - makes the beam brighter, this is sometimes linked to the on/off control
X shift - moves the spot across the screen
Y shift - moves the spot up and down the screen
X gain - magnifies movement in the X direction
Y gain - magnifies movement in the Y direction
Focus - obvious
Time base - makes the spot move from one side of the screen to the other at various speeds. Usually from 1 second to 1 millionth of a second for one crossing.