"Actors" are VTK components that allow for the adjustment and control of the apperarance properties of the physical manifestations of the data as rendered onto the screen. Some of the properties typically controlled via actors are transparency and color mapping. ( The term "actor" comes from analogy with the stage - The actor is a physical representation of the data, "standing" on the "stage" ( appearing in the rendering window ), whose appearance can be modified through lighting, makeup, costumes, etc. ) Another term for actors is "props"; The distinction between actors and props ( if there is any ) is not clear.
See page 285 of the VTK Users Guide for a long list of useful VTK actors. Some of the ones that were used in this tutorial's sample programs are discussed here: