Before talking about screen resolutions on iOS devices, it is important to understand the difference between a pixel and a point.
A pixel is a measure of the size of the screen, or the size of the bitmap in an image (icon, button, and so on) that you create.
A point is an area on the screen where drawing occurs.
For a long time, a pixel and a point were the same size, and very few game developers were concerned about them being different.
When Apple introduced the retina display in the iPhone 4, everything changed because with the retina display, a point is two pixels (the resolution doubles in both the x and y directions). So, while the number of pixels on the display increased, the number of points ...