Rapid variation of voltage, even over a relatively small range, can cause lights to flicker and can cause certain types of equipment to malfunction. These issues need to be investigated on a case by case basis as the causes and solutions are many and varied.
With voltage fluctuations the voltage does not necessarily vary through too great a range, but the variation in the voltage occurs quickly and often repetitively. Sources of such problems can for example be arc welders, or motors with repetitive start patterns. The solutions to mitigate such problems very tremendously and are dependant on what is causing the problem and therefore need to be investigated on a case by case basis.