All the HTML content between the opening and closing layer tag can be treated as a single item of content that can be moved and altered in various ways. You can use inline JavaScript to dynamically change whether a layer is visible or not, so that you can hide a layer and make it reappear again. And you can position layers relative to each other, such as having the top of one layer start just below the bottom of another.
Layers have a stacking order.
Layers can be transparent or opaque. Underlying layers show through transparent layers, but not through opaque ones. You can specify background images and background colors. If you set the background image or color, the layer will not be transparent, and it will obscure any layers that lie below it. Layers can be nested inside layers.