Cycles is Blender’s physically-based production renderer.
To use Cycles, set it as the active render engine in the Render properties. Interactive rendering can be started in the 3D viewport by changing to the Render display mode. To render an image or animation, use the Render menu.
Cycles may be able to use your GPU for faster rendering. To see if and how you can use your GPU for rendering, see the documentation on GPU Rendering.
The source code is available under the Apache License v2, and can be integrated in open source and commercial software. Cycles is natively integrated in Blender, Poser, and Rhino.
Cycles website with more information and a gallery.