What is a discrete GPU and why use it?

What it is:  Gaming laptops have two graphics cards, the "onboard" graphics card and a separate GPU dedicated for gaming graphics. Discrete graphics is a GPU that is separate from the CPU.  

Discrete graphics have dedicated memory that is not shared with the CPU. 

Why/when is it used: Typically, when a laptop is used for gaming or when connecting a monitor to the onboard graphics instead of PCI.

Most common symptom, particularly on a laptop:  When loading into RedM, the screen is black. 

Other symptoms: You may experience high ping, freezing, RAGE error crashes, etc.

Resolution:  Configure discrete GPU with the instructions below.