Please see this for a very nice introduction to VTK, and especially the terminology
VTK conceptually divides the "pipeline" into two segments: A data processing segment and a rendering segment.
Data Processing Segment
The data processing segment consists of data sources and data filters. These objects give the user direct access to the data.
A filter is an object that takes one or many inputs, operates on the inputs, and produces one or many outputs.
A source is a special case of a filter where there are 0 inputs.
The rendering segment is essentially an OpenGL wrapper. In this segment of the pipeline, the user does not have direct access to the data - it is in a form that is most efficient for the graphics hardware. The rendering segment consists of mappers, actors, renderers, and render windows.
Mappers serve as the transition between the two segments of the pipeline. It can be thought of as a filter that converts data in a manipulatable form (e.g. something from the data processing segment) to a form that can be rendered.
An actor is an "OpenGL object". That is, if you have a geometric object or a model of some sort, you will have to create an actor for it to give to the renderer.
This is the actual window that the operating system opens.
A render window can contain one or many renderers. The renderer is where the data is actually drawn to the screen.