STN, or super-twisted nematic, is a display technology that utilizes liquid crystal display technology.  All LCDs utilize some for twisted nematic scheme, which means that liquid crystal molecules modify the angle of incoming polarized light and output it at a different angle.  For a standard twisted nematic device, like most computer monitors, that angle is a 90-degree delta from input to output.  For a super-twisted nematic graphics module, that angle is usually in between 180 and 270 degrees, which offers certain advantages over the less pronounced angled crystals.

For TFT LCD displays, there is not a narrow threshold for crystal realignment between the passive and electric field states.  In some applications more direct control is desired, such as passive-matrix addressing.  This methodology relies on elements maintaining a desired display without the need to actively drive the elements with a voltage of some form.  LCD monitors for computers use and active addressing, meaning the pixels lose their display modulation when they are not actively being powered.  A similar situation happens within the realm of computer memory, where there is volatile memory which must be constantly refreshed to maintain its contents, like computer RAM.  There is also non-volatile memory, like hard drives or thumb drives, which can maintain memory contents passively without any power source.