A screening machine consist of a drive that induces vibration, a screen media that causes particle separation, and a deck which holds the screen media and the drive and is the mode of transport for the vibration.
There are physical factors that makes screening practical.  For example, vibration, g force, bed density, and material shape all facilitate the rate or cut.  Electrostatic forces can also hinder screening efficiency in way of water attraction causing sticking or plugging, or very dry material generate a charge that causes it to attract to the screen itself.
As with any industrial process there is a group of terms that identify and define what screening is.  Terms like blinding, contamination, frequency, amplitude, and others describe the basic characteristics of screening, and those characteristics in turn shape the overall method of dry or wet screening.
In addition, the way a deck is vibrated differentiates screens.  Different types of motion have their advantages and disadvantages.  In addition media types also have their different properties that lead to advantages and disadvantages.
Finally, there are issues and problems associated with screening.  Screen tearing, contamination, blinding, and dampening all affect screening efficiency.