Direct Specular-
This is the shininess of the objet or material and appears as a grey scale render of highlights on your object. This is typically added to the Diffuse with a Plus Merge Node. 

Indirect Specular-
This highlights or shiny aspects of the model on nearby areas, models or parts of the model itself. This appears identically to the Specularity but will appear on other parts of the object having bounced off your object. 

Refraction-
The passage of light through an object and how it bends and moves the light rays
Reflection-

This is the reflections of objects and environments in the shiniest aspects of the your model and is visible as bright components that can be added or (+) merged on to your model.
SSS-
This render function looks at how light enters components of the model and bounces back out. This allows for waxy renders and can be used to mimic the way light can appear through thin areas of flesh. 

Can be used for wax and thin plastic substances and can be driven by a SSS map where by black inhibits light transmission and white allows light through. 

Grey pixels much like alpha, will allow gradients of influence.