Does it have viruses?
What does it do better than Blender?
Why do you like it?
Google it. --> http://www.yafray.org/
Blender has rendering tools built in: a scanline renderer and a simple raytracer.
Yafray (Yet Another Free RAYtracer) is a more powerful raytracer and radiosity engine. So when you see people posting artwork next to words like HDRI, Global Illumination, Caustics, and DOF, you’re looking at something rendered using Yafray.
Blender is designed so as to make it easy to use Yafray in particular. This is not uncommon. For example, Softimage|XSI uses the Mental Ray raytracer. It’s so tightly integrated you don’t even realize that it’s a separate program. Blender’s not quite as tightly integrated with Yafray, in part because you can set up Blender to use other raytracers as well (like POV-ray), but it is often considered part of the package just for the convenience and ease of use.
People like it just because they get to play with light better than Blender can on its own. Better lighting == better pictures.
Hope this helps.
Yafray is a ray tracer. A ray tracer has more realistic lighting than the internal scan line renderer at a cost of higher rendering times. I would suggest using the internal renderer until you run into limitations that the internal renderer can’t handle.
Does it have viruses?
You must be from the windows world where free (as in cost) software oftain contains spyware/adware etc. Yafray is an open source project like blender (free as in freedom). You have the ability to download the source code and see exactly what it is doing, modify it and give it to your friends as long as you also give them your changes in source code form. Since everyone has access to the source code and can see what it is doing it is almost impossible to hide malicious code.
GreyBeard