Volume rendering is a rendering approach that allows high-quality, integrated rendering of volumetric effects like smoke, clouds, spray, and fire.
Volume rendering is suitable for rendering many types of volumetric effects such as:
Detailed "hero" clouds, smoke, or fire
Fields of instanced clouds, smoke, or fire
It is easy to create volumes from primitives without invoking the fluid solver.
In this particular example, a primitive torus is used to render some smoke volume. Using an IsoOffset SOP produces a volume that fills the interior of the torus. Then, a material using a Volume Cloud is assigned to the volumetric data of the torus shape. Setting the Smoke Cloud Density to 5 and the Smoke Shadow Density to 10 helps create a more smoke-like look and feel.
Here is the torus rendered with tweaks to the volume step sizes (in the Mantra Render Operator), shadow map quality (under Depth Map Options of the spotlight), and volume primitive divisions (on the IsoOffset SOP). The smoke Diffuse color was adjusted too.