Selection Basics Using masks rather than the selection tools is somewhat like working in grayscale instead of bitmap mode.
Masks offer much finer control of how filters, effects, and colors are applied.
To continue the analogy, in bitmap color mode, a pixel is either black or it is white. In grayscale, you have 256 levels of color, ranging from black through gray to white. With a basic selection, a pixel is either inside the selection marquee or outside. Two simple marquee selections have been made, without feathering or anti-aliasing. When you make a basic selection and, for example, apply a color, those pixels inside the selection receive the color, those outside do not.