Hairtrix is the offspring of two other plugins, HairFX and Ornatrix. Both reasonable hair styling and rendering tools, but both lacking something that the other had. So the obvious choice was to get these kids together and have a child.
HairFX holds its strength in the generation of fur. You can quickly setup a fur system; point vectors this way and that, and you suddenly have a plush toy. The hair/fur is atmosphere based, so it’s generated at rendertime. There are model hairs, or guide hairs, to designate hair styles. HairFX interpolates between the hairs, and fills in the gaps. But for more sophisticated hair stuff, we must move onto the ‘trix’ side of Hairtrix.
The Ornatrix tools add a whole ‘nother realm of control to your hair styles. Combing and styling the hair is just one. You have controls over clumping of the hair through maps, strands and how close the hairs will gather to the guide hairs. You have a braid generator. Channels control provides even deeper control of the strands by being able to separate strands or clusters, or having dynamics affect different parts of the strand differently.
Both HairFX and Ornatrix atmospherics and shaders are available to use, but it seems that the Ornatrix shaders work more naturally with lighting, giving nice highlights, shadows and backlighting.
The two plugins can be used individually, but are best utilized together by quickly setting up the hair style in HairFX and then stacking the Ornatrix modifiers on top of it for the more advanced processes.
One critique I have for Ephere/Turbosquid, is to get a better 3D artist to generate the imagery for the manual. The images certainly do not do the tools justice.