Hiding and deleting a mask
If you take a look at the layer palette you will notice a little mask symbol appearing thext to the layer name. This will indicate that a mask is used on the layer. If you let your mouse hover over the symbol you can even see what the mask looks like

You can choose to see the layer without the mask applied by clicking the mask tab on the top line, and then clicking the mask that appears on the line of the layer. When the mask is crossed over you can see the layer without the mask, like this:

To go back to 'appearance' mode again, click the glasses on the top line next to the mask.

If you want to delete a mask, choose Masks->Delete. If you answer yes to merging the mask into the layer, the transparent areas will be deleted for good and you can not go back. If you say no, the entire mask will vanish and the layer will be as when you started out.
These are the basic techniques. You can also create a new mask from a selection, another open image, or simply create it from scratch choosing 'view all' or 'hide all' and then edit the mask in mask edit mode to get the look you want.

>>> next page, Example: scanned drawing>>>