A tap changer is a mechanism used with transformers which allows for variable turn ratios to be selected or adjusted in discrete steps.  Transformers with this mechanism obtain the variable turn ratio by connecting to a number access points known as taps along either the primary or secondary winding.  A typical tap changer usually possesses tens of taps and allows for a certain percentage of variation (for example, ±10%) from the nominal transformer rating which, in turn, allows for stepped voltage regulation of the output. 
Tap changers exist in two primary types: no load tap changers (NLTC) which must be de-energized before the turn ratio is adjusted and on-load tap changers (OLTC) which may adjust the turn ratio during operation.  For many power transformer applications, a supply interruption during a tap change is unacceptable, and the transformer is often fitted with a more expensive and complex OLTC mechanism.  OLTCs may be generally classified as mechanical, electronically assisted, or fully electronic.  However, existing OLTCs normally utilize analog means to adjust the turn ratio and indicate the current selection of the turn ratio.  The analog means is usually weak in terms of precision and functionality.