Manufacturers like bragging about higher numbers... specially phone manufacturers. 4", 5", 7" screens... HD, 720p, 1080p... but what do all these numbers REALLY mean?
For example, the iPhone has a 3.5" screen. Which a lot of the Android phones are already boasting 4" and larger. Does this make them better? Not necessarily. There are 3 sets of numbers that are very confusing, if not misleading, when comparing screens.
First is resolution. This is the number of dots or pixels you screen has in total.
Second is screen size. This is the actual physical measurement of your screen taken diagonally from corner to corner.
And third is destiny...er... density. This is the number of pixels/dots per inch. Simply referred to as ppi or dpi, it's this last one that makes things a bit confusing. Larger screens that have lower ppi are cheaper than smaller screens with higher ppi.
Just as a comparison, I've resized a photo to show the exaggerated similarities and differences between these 3 properties.
For the first example, these 2 photos have the same physical screen size: 5.5" screen
5.5" screen at 25 ppi
5.5" screen at 100 ppi
For the next example, the next two photos have the same resolution: 100 x 100 pixels
100 x 100 resolution on a 5.5" screen
100 x 100 resolution on a 1" screen
For the last example, the two photos show the same ppi / dpi: 100 ppi / dpi
100 ppi on a 5.5" screen
100 ppi on a 1" screen