Which resolutions does the Retina MacBook Pro support, and how?
Supported resolutions


  
  Retina, Ratio 16:10
  
  
  2880 by 1800 pixels
  
  scaled, Ratio 16:10
  
  
  1920 x 1200 pixels
  1680 x 1050 pixels
  1280 x 800 pixels
  1024 x 640 pixels
  
  
  
  According to the official tech specs by Apple.


How does the scaling work?

According to an article in AnandTech:


  Selecting any of these options gives you the effective desktop resolution of the setting, but Apple actually renders the screen at a higher resolution and scales it to fit the 2880 x 1800 panel. As a result of this scaled rendering, there can be a performance and quality impact. 


There is a new option to configure the resolution in the system settings. While you actually change the resolution, Apple only refers to the visual impact of the settings.

So basically changing the resolution offers:


Large text, but much crisper.
More space for visual content (and smaller text), but not as sharp.
Something in-between (default retina).




(original picture from AnandTech)

Note:

Due to the imprecise definition of "retina display", this answer may seem confusing, because at the top Apple speaks of 2880x1800 as retina, but the picture suggest something different.