Each object is represented by a material surface. These objects are sized in device-independent pixels (or dp, for short). This is a great unit to measure user input because it allows designers to design interfaces independent of the screen size.
It can also be converted to absolute units (such as inches or millimeters) depending on the device's screen size.
Note
You can learn more about dp at http://en.wikipedia.org/wiki/Device_independent_pixel.
Surfaces are thought of as 3D objects, having width, height, and depth. All surfaces have a depth of 1 dp, but they can have any width or height. Surfaces also overlap, so they have a vertical offset from each other.
This vertical offset (or elevation) allows layering, creating a sense of depth similar to the real world. The content on a surface, such as typography and images, lies flat on the surface. Think of it like ink on paper, where the paper has a depth that's easier to notice than the ink printed on it.
This layering effect is accentuated...