Question: According to Android definitions, I've found that:

* Resolution: The total number of physical pixels on a screen
* Density: Based on the screen resolution, the spread of pixels across the physical width and height of the screen.

How can I understand better the differences between the two definitions?

Answer: 
Resolution is about how many pixels you can show on screen.

Density is based on your device real size, if it's small and has a higher resolution, than the density is high cause you show more pixels in less physical space.
Basically:

1. **Resolution** concerns an *absolute* number of pixels. (check out wikipedia [Image Resolution](http://en.wikipedia.org/wiki/Image_resolution))
2. **Density** (aka Pixels per inch - PPI) concerns a *relative* number of pixels per inch. (check out the wikipedia [Pixel Density](http://en.wikipedia.org/wiki/Pixel_density))

Take care,
Beco