YafaRay is a raytracing engine. Raytracing is an image rendering technique for generating realistic images by tracing the path of light through a 3D scene. A render engine consists of a "faceless" computer program that interacts with a host 3D application to provide specific raytracing capabilities "on demand". Blender 3D is the host application of YafaRay.
Tracer is a package for ray-tracing in Python. Its main application area is solar energy, and the architecture focuses on extensibility and programmability, so it can be used as a part of other programs. For example, it could be used for optimization algorithms or annual performance simulations.
Protracer is a simple raytracer originally written in 1999 as a university course project. Please be aware that, despite the name, there is nothing especially professional about Protracer. The name is meant to be a pun on the Protracker music tracker application. It currently supports a very limited subset of the POV-Ray file format.