In order to develop for Android you need to take into consideration that in the market there are devices of multiple manufactures, each of them with their specific screen characteristics.
Android OS groups device’s screens in categories depending on their size and on their pixel density.
There are four categories depending on the physical size of the screen (xlarge, large, normal, small) and four groups depending on the pixel densities (xhdpi, hdpi, mdpi, ldpi).
Android has the ‘dp’ unit do designate dimensions in a density independent fashion. If you design your layouts use always ‘dp’ units for dimensions instead of pixels.
If you will support all screen densities, you might need to have four versions for each Drawable. It is nice practice to have different layouts depending on the physical dimensions too, as a large screen could have room for showing more controls than a smaller screen for instance.
It is advisable to take the full reading of the multiple screens support from Android site, which is excellent.