"Mappers" are VTK components that receive data from other components, ( usually filters, but sometimes directly from sources ), and "map" the data to some sort of a physical manifestation that can be rendered by the rendering engine.
Mappers are easy to confuse with filters. In order to understand the difference, one must know that VTK conceptually divides the pipeline into two segments: A data processing segment consisting of sources and filters, and a rendering segment, consisting of actors, renderers, and windows. Mappers serve as the transition between the two segments of the pipeline. So if the output of a component is being used as the input to an actor, then it is a mapper, but if the output is being read by some other type of component ( either a mapper or another filter ), then it is a filter.
See page 284 of the VTK users guide for a long list of useful VTK mappers. Some of the ones that were used in this tutorial's sample programs are discussed here: