The Pathtracer node is simply the most accurate and powerful raytracing node.

To understand the Pathtracer node it should be thought of as the following raytracing nodes combined in the one node:

RT Multi-Bounce Reflections
RT Glass Refraction
RT Diffuse Bounces

Settings

The three main settings in the pathtracer are the Max. Diffuse Depth, Max. Glossy Depth or Max Refraction Depth.

Max. Diffuse Depth

This will control the number of bounces light will be bounced around the scene, the higher the number of bounces the more the light in the scene will be bounced.

Max. Diffuse Depth set to 1 Max. Diffuse Depth set to 6

Max. Glossy Depth

This will control the number of reflection bounces. Set to one and the scene will be reflected once, so reflections won’t show reflections on reflected objects.
Set to two and above then reflective surfaces will show up in the reflections of other reflective surfaces.

Max. Glossy Depth set to 1 Max. Glossy Depth set to 11

Max Refraction Depth

This will set the number of refractions on transparent objects. Set lower and you will only refract through one transparent object, set higher will refract through more transparent objects.

Max Refraction Depth set to 2 Max Refraction Depth set to 8