When producing applications for a desktop or laptop computer, we don't have to give too much thought on the actual screen real estate we have to work with, or the Pixels Per Inch(PPI) resolution for that matter. It can be generally assumed that we will have at least a 1024x768 screen to work against, and we can be sure that it is a 72 PPI display. With mobile, that it all out the window.
With mobile device displays, our applications can basically be full screen or almost full screen; that is, but for the notification bar. These device screens can vary in size from just a few pixels, to hundreds. Then we must take into account different aspect ratios and the fact that the screen will certainly display 250 PPI or above. We must have a new set of checks in place to perform application layout modifications depending upon the device.