Quick Mask will allow you to create your own complete mask on a picture or subject or it will allow you to add to a selection made by any of the selection tools.
A practical example is probably the best way to show this tools capabilities:
If you look at the picture above, we can quickly make a mask of this sign and convert that to a selection via Quick Mask. So, go to the bottom of the tools palette and click on the Quick Mask tool icon or you can make it active by going to Select>Edit In Quick Mask Mode. When active, select one of your paint tools and paint on the surface of your image, going over the part you want to include in your selection. As you paint on the surface, a colour overlay will show you the area you are covering. This can be changed in the options palette along with the other option of selecting a masked area or a selected area. Double click the Quick Mask icon for this option palette.
You can change the size of your brush by right-clicking on your image and moving the
Master Diameter slider to the left or right, depending on if you want a smaller or larger brush. You can also use the Magnifying tool to zoom in, making it easier to select the edge of your selection. To zoom in, either select the Magnifying tool and click on the image or press Cntrl and + on your keyboard (if using a PC). To zoom out press - instead of + or click the tool which has the - symbol in it in the top tool bar. If you go wrong, you can switch the foreground colour to white and paint over the part that shouldn't be selected to remove it. Or click on your history tab and go back however many steps you need to to correct the selection. You can also hit Cntrl + Z on your PC's keyboard to go back just one step. Try making more, smaller brush strokes than one long one, that way you'll have less to undo and paint over again if you do go wrong.
Once you have completed your quick mask, click the Quick Mask icon again and your mask will be changed to a selection as shown below.
You can either copy your selection to another layer (Edit>Copy) (Edit>Paste in your new layer) or remove the background by going to Edit>Cut. If you have a layer called Background, you'll need to remove it or you won't cut to a transparency when you make your changes. In other words, it will look like your image is sat on a white background instead of nothing as the below images show:
Left: On a white default background. Right: Background removed.
You can also use Quick Mask to repair or improve selections made with any of the selection tools. For example, if you find that the selection tools like the magic wand tool does not select all of the area you want, switch to quick mask. The masked or selected area will show as a red transparent overlay as shown previously. As before, black will add to the selection and white will remove parts of it then when it's complete, hit the Quick Selection icon to return to the selection.