Some other important things to know about layers:
If you right click on the "Background" button on the Layers Palette of a new graphic you have opened (such as a photo), you can choose "Promote to Layer". This will make it a layer and enable the opacity slider. But, you now have nothing under your photo (like a white background). To correct this, just add a new layer, name it Background and drag it under the new layer.
Always name your layers. If you don't, you will probably get really confused.
You can view what is in a layer by placing the cursor over the layer name in the Layers Palette. This will pop up a box with a rendering of the layer. flower bangbros.com

The dark and light grey grid you see behind graphics on layers means that part of the graphic is transparent. When you create a new layer, it is automatically transparent.
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Learn to "Paste as a New Layer". If you make a selection in one graphic and paste it into another graphic "as a new layer", the background is automatically transparent.
When you have something on a layer, it is automatically selected. Say you add text to a layer and you deselect it. You can add a drop shadow or bevel the text without selecting it. By its nature, a layer is selected unless you make a specific selection of parts of the layer.flower bangbros.com