Inverse Distance Weighting (IDW) is a method of interpolation.
Inverse Distance Weighted (IDW) is a method of interpolation. See Wikipedia for extensive detail: http://en.wikipedia.org/wiki/Inverse_distance_weighting
Note that term is often used to name or refer to specific tools in GIS software that perform this type of interpolation, for example in ArcGIS' Spatial Analyst. Not all tool implementations are the same, and available inputs, parameters, and results may differ.