A layer is a work sheet on which many editing functions may be made such as drawing, painting, erasing, pasting images and much more. An image may be constructed of many layers, one on top of another, or one single (default) layer.
If a picture has a floating selection, it will appear as a layer in the Layers window in such a way that will enable you to perform on it some of the operations that can be performed on layers (such as changing places amongst the hierarchic levels of the layers and changing its transparency).
The Active Layer's Properties (3)
This part of the Layers window contains indexes (Tabs) containing the properties of the active layer and its Thumbnail as well as that of its mask (if one exists).
Layers Properties Tab - This card-index contains the Thumbnail of the active layer and that of its mask (if one exists). The card-index also displays the name of the active layer, its Visibility Mode, its level of Opacity and its Blend Mode.