Pretty much everything about this and how to achieve the best support for multiple screens of different sizes and densities is very well documented here:
- Supporting Multiple Screens
Actual physical size, measured as the screen’s diagonal.
For simplicity, Android groups all actual screen sizes into four
generalized sizes: small, normal, large, and extra-large.
The number of pixels within a physical area of the
screen; usually referred to as dpi (dots per inch). For example, a
“low” density screen has fewer pixels within a given physical area,
compared to a “normal” or “high” density screen. For simplicity,
Android groups all actual screen densities into six generalized
densities: low, medium, high, extra-high, extra-extra-high, and
The orientation of the screen from the user’s point of
view. This is either landscape or portrait, meaning that the screen’s
aspect ratio is either wide or tall, respectively. Be aware that not
only do different devices operate in different orientations by
default, but the orientation can change at runtime when the user
rotates the device.
The total number of physical pixels on
a screen. When adding support for multiple screens, applications do
not work directly with resolution; applications should be concerned
only with screen size and density, as specified by the generalized
size and density groups.
Density-independent pixel (dp)
pixel unit that you should use when defining UI layout, to express
layout dimensions or position in a density-independent way.
The density-independent pixel is equivalent to one physical pixel on a 160
dpi screen, which is the baseline density assumed by the system for a
“medium” density screen. At runtime, the system transparently handles
any scaling of the dp units, as necessary, based on the actual density
of the screen in use. The conversion of dp units to screen pixels is
px = dp * (dpi / 160).
For example, on a 240 dpi screen, 1 dp
equals 1.5 physical pixels. You should always use dp units when
defining your application’s UI, to ensure proper display of your UI on
screens with different densities.
If you are at all serious about developing an Android app for more than one type of device, you should have read the screens support development document at least once. In addition to that, it is always a good thing to know the actual number of active devices that have a particular screen configuration.
- Screen Sizes and Densities