Что такое пиксель и точки в iPhone?

из ссылки UIImage,

size

The dimensions of the image, taking orientation into account. (read-only)

@property(nonatomic, readonly) CGSize size

Discussion

In iOS 4.0 and later, this value reflects the logical size of the image and is measured in points. In iOS 3.x and earlier, this value always reflects the dimensions of the image measured in pixels.

В чем разница между пикселем и точками?