For use in a computer, the tube is required only to function as a switch having an on and an off state. For audio use however, the situation is somewhat different. Many musicians, especially Guitar players, like the 'warm' sound of tubes. Indeed, the unique characteristics of a vacuum tube to tend to 'colour' music played through them.
Lets consider a simple vacuum tube circuit as below.
The tube works by glowing a filament red hot in which case electrons are freely emitted. The negative electrons flow towards the positive plate and hence current flows through the tube. In order to control the flow of electrons between the filament and the plate, a screen grid is introduced into the tube. By applying a potential to this grid, electrons may be attracted towards or repelled away from the plate. Using our simply test circuit above, we may investigate the linearity of this tube.