How to Never Delete Important Layer in Adobe Photoshop

Adobe Photoshop allows you to delete any layer including and Background. Padlock on the right side of Background layer does not mean you can not delete it, it just mean you can not move it around or in another open document. To protect any layer including Background from accidental deletion you must use Lock all feature which is represented with padlock icon and seats on top of layer stack. When layer is fully locked you can not delete it in any case even if you try to delete group of layers with locked layer inside it. To lock any layer you must first select it then to click Lock all icon in Layers panel. To lock fully Background layer you must first convert it to normal layer.

Lastly how to recognize if layer is Background protected or fully locked? Beside you won't probably name your layer Background, fully locked layer  in CS6 is represented with normal padlock icon (at least for my money) and Background with inverted. In previous versions fully locked padlock is darker. Another indicator is padlock on top of layers stack in Layers panel highlighted when Background layer is selected. Same recognitions are applicable to recognize if any of layers is fully locked or it have locked only pixels, transparency or position.

Questions you may have:
Can I lock any layer? Yes.
Can I lock group of layers? Yes you can. Select group then click Lock all.

Reference:
Photoshop / Moving, stacking, and locking layers

Product used in this tutorial: Adobe Photoshop CS6