Adobe Photoshop uses masks to isolate and manipulate specific parts of an image. A mask is
     like a stencil. The cutout portion of the mask can be altered, but the area surrounding the cutouts
     is protected form change. You can create a temporary mask for on-time use, or you can save
     masks for repeated use.
     In this lesson, we will cover the following:

• Refine a selection using a quick mask.
• Save a selection as a channel mask.
• View a mask using the Channels palette.
• Paint in a mask to modify a selection.
• Create and use a gradient mask.

     In Adobe Photoshop, you can make temporary masks, called Quick masks, or you can create      permanent masks and store them as special grayscale channels, called Alpha channels.
     Photoshop also uses channels to store an image's color information and information about
     spot color. Unlike layers, channels do not print. You use the Channels palette to view and work
     with alpha channels. ImageReady does not support channels except for alpha channels used
     for PNG transparency and weighted optimization.

-From Adobe photoshop 6.0-

     Creating a quick mask

     First save images.      

     Next Launch Photoshop , Go to File > Open-Crash.jpg the image that you just saved.
                                
     Next Open the two image files, Fan.jpg and refin.jpg.
     Minumize the refin.jpg .
     Copy Layer the Fan image, Drag using the MoveTool the Copy layer of the Fan.jpg onto the      working image of crash.
                                             
     

     Go to the layers palette and click on the Mask icon at the bottom of the palette or on the      bottom of the tool bar.
                                             
      Now choose the Gradient tool and Go to the Gradient Option bar and select the upside down       triangle(Foreground to Background).
                
      Now Drag the Gradient from the right side to the middle (if the fan image is still covering to much       of the crash dumby then don't go as far to the middle as you did the first time), this is what it       should look like.
                                     
      Once you got it, Now Drag the last image (refin) in to the working area.
                                     
      Do the same process as we did on the fan image.

Select a mask Tool.

Select the Gradient tool and Options ( Foreground to Background ).

Drag from the left side to the middle this time.
                       
     The cool thing about this project is that the crash image is on the bottom layer.
     Quick masks are temporary. They disappear when you deselect. However, any selection can be      saved as a mask in an alpha channel. Think of alpha channels as storage areas for information.      When you save a selection as a mask, a new alpha channel is created in the channels palette. ( An      image can contain up to 24 channels, including all color and alpha channels.) You can use these      masks again in the same image or in a different image.

   In addition to the temporary masks of Quick Mask mode, you can create more permanent masks by storing    and editing selections in alpha channels. You create a new alpha channel as a mask. For example, you can    create a gradient fill in a blank channel and then use it as a mask. Or you can save a selection to either a new    or existing channel.

   An alpha channel has these properties:

     Go to Select > Load Select.                                   Then in this window click OK.
                                           
      This was the last mask that we made so it's selected, now we can save it. To do this Go back to       Select > Save Selection.
              
      Click on OK, and in the channels palette there is now a new channel called alpha 1, which we
      just created.
                    
                                                                                          Saved mask

      Editing a Mask 

     You can use most painting and editing tools to edit a channel mask.

• Hide all channels except for alpha 1 and the screen is now a gray scale
• Painting with white erases the mask and increases the selected area.
• Painting with black adds to the mask and decreases the selected area.
• Painting with Gray values adds to or subtracts from the mask in varying opacity, in proportion
  to the level of gray used to paint.