Composites are simply combining multiple images to create one better than any of the multiple images used to create it.  Layers in Photoshop allows building of multiple image with various levels of transparency to create one image that tells a story.

Layering is in fact, compositing.  Layers such the three combined in the Seabirds photo can be combined with the Boca Chica Bridge composite where I used only two sections of two different photos, as well.