YafaRay is a free and open source raytracing engine. Raytracing is a rendering technique for generating realistic images by tracing the path of light through a 3D scene. I have no idea what a raytracing engine means or how critical a client it is to Blender. But this video showcasing different capabilities of YafaRay is an absolute delight to watch.
Thanks to blendernation for the video. If you like Blender, then you should definitely watch these breathtaking Blender made movies which you probably haven't seen before. And if you like some real learning, read this carefully sorted collection of Blender tutorials.