Layer Basics in Adobe Photoshop CC (2015 release)
This excerpt references lesson files, which are available with the purchased book.
Every Photoshop file contains one or more layers. New files are generally created with a background layer, which contains a color or an image that shows through the transparent areas of subsequent layers. All new layers in an image are transparent until you add text or artwork (pixel values).
Working with layers is analogous to placing portions of a drawing on clear sheets of film, such as those viewed with an overhead projector: Individual sheets may be edited, repositioned, and deleted without affecting the other sheets. When the sheets are stacked, the entire composition is visible.