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 a measure of how many pixels a display can show at a time eg. 640x480. Density is a measure of how big each pixel is in actual units. This relates heavily to screen size eg. 640x480 on a 15' display. 

When comparing one device's resolution to another, it's a straight-forward comparison: the higher the resolution, the more information you can display at a time. When comparing density: the higher the density the sharper the display will appear. 

If you have high resolution and low density, then your screen will be very large. On the other hand, low resolution and high density will result in a very small screen.
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