This shader is considered deprecated and will be removed in a future release. You should use the Standard Hair shader instead.
Image credit to Pascal Floerks (rollover image)
The Hair shader is designed to render the look of hair or fur. Note that this shader needs to be used on geometry that has been defined to look like hair or fur (for example using Arnold's curves shape node). Other shaders such as Lambert could be used, but the Hair shader has a specular shading model specifically designed for thin hair fibers and lets you define different diffuse colors for the root and tip of the hairs, for example. This shader currently uses a simple Kay-Kajiya shading model.
Further examples of customer work using the Hair shader can be found here.
The controls are split into groups for Diffuse and Specular
The images below show the separate effect of the root, tip and specular colors. The far right image is the resulting combination of these colors.
The hair shader is not designed for hairs that are much bigger than the pixel size.
When rendering hair with motion blur, the hair must first be cached, otherwise you will get incorrect results. The animation below shows the results of rendering with and without hair caching (rollover mouse to view the animation).
Left: Cached hair (motion blur renders correctly). Right: Hair without cache (motion blur renders incorrectly). Rollover mouse to view the animation.