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.
OpenXL is a multiple purpose C open engine. It is a proceduralization library. It can be seen as the artist, the designer, or even the programmer of your project. It is designed so that it has some function power that could turn into any task (performing a texture, a pixel art image, a mesh, a model, a level, a song, etc.). The idea is to play the part of the missing components on your team.