When you select part of an image, the area that is not selected is masked, or protected from editing. So, when you create a mask, you isolate and protect areas of an image as you apply color changes, filters, or other effects to the rest of the image. You can also use masks for complex image editing such as gradually applying color or filter effects to an image.
A. Opaque mask used to protect the background and edit the butterfly B. Opaque mask used to protect the butterfly and color the background C. Semitransparent mask used to color the background and part of the butterfly
Masks are stored in alpha channels. Masks and channels are grayscale images, so you can edit them like any other image with painting tools, editing tools, and filters. Areas painted black on a mask are protected, and areas painted white are editable.
Use Quick Mask mode to convert a selection to a temporary mask for easier editing. The Quick Mask appears as a colored overlay with adjustable opacity. You can edit the Quick Mask using any painting tool or modify it with a filter. Once you exit Quick Mask mode, the mask is converted back to a selection on the image.
To save a selection more permanently, you can store it as an alpha channel. The alpha channel stores the selection as an editable grayscale mask in the Channels panel. Once you store the selection as an alpha channel, you can reload it at any time or even load it into another image.
Note:
You can mask or hide parts of a layer using a layer mask.
You can create a new alpha channel and then use painting tools, editing tools, and filters to create a mask from the alpha channel. You can also save an existing selection in a Photoshop image as an alpha channel that appears in the Channels panel. See Save and load selections.
at
the bottom of the Channels panel.To change options for an existing channel, double-click the channel thumbnail in the Channels panel or select Channel Options from the Channels panel menu.
Options available in the New Channel and Channel Options dialog boxes:
Masked Areas
Sets masked areas to black (opaque) and selected areas to white (transparent). Painting with black increases the masked area; painting with white increases the selected area. When this option is selected, the Quick Mask button in the toolbox becomes a white circle on a gray background 
.
Selected Areas
Sets masked areas to white (transparent) and selected areas
to black (opaque). Painting with white increases the masked area;
painting with black increases the selected area. When this option
is selected, the Quick Mask button in the toolbox becomes a gray
circle on a white background 
.
Color
Sets the color and opacity of the mask. Click the color field to change the color. The color and opacity settings affect only the appearance of the mask and have no effect on how underlying areas are protected. Changing these settings may make the mask more easily visible against the colors in the image.
When the new channel appears at the bottom
of the Channels panel, it is the only channel visible in the image
window. Click the eye icon 
for
the composite color channel (RGB, CMYK) to display the image with
a color overlay showing the mask.
-
Select the brush or an editing tool and do one of the following to add or subtract from the mask created from the alpha channel:
To remove areas in the new channel, paint with white.
To add areas in the new channel, paint with black.
To add or remove areas using opacities less than 100%, set the Opacity in the options bar of the painting or editing tool and then paint with white or black. You can also paint with a color to achieve lower opacities.
You can save any selection as a mask in a new or existing alpha channel and later reload the selection from the mask.
You can use a selection as a layer mask by loading the selection to make it active, then adding a new layer mask.
at
the bottom of the Channels panel. A new channel appears, named according
to the sequence in which it was created.You can reuse a previously saved selection by loading it into an image. You can also load the selection into an image after you finish modifying an alpha channel.
-
Select the alpha channel, click the Load Selection button
at the bottom of the panel, and then click the composite color channel near the top of the panel.Drag the channel containing the selection you want to load onto the Load Selection button.
Ctrl-click (Windows) or Command-click (Mac OS) the channel containing the selection you want to load.
To add the mask to an existing selection, press Ctrl+Shift (Windows) or Command+Shift (Mac OS), and click the channel.
To subtract the mask from an existing selection, press Ctrl+Alt (Windows) or Command+Option (Mac OS), and click the channel.
To load the intersection of the saved selection and an existing selection, press Ctrl+Alt+Shift (Windows) or Command+Option+Shift (Mac OS), and select the channel.
Note:
You can drag a selection from one open Photoshop image into another.
Note:
If you are loading a saved selection from another image, make sure to open it. Also, make sure your destination image is active.
-
Select an Operation option to specify how to combine the selections if the image already has a selection:
Intersect With Selection
Saves a selection from an area intersected by the loaded selection and existing selections in the image.
Note:
You can drag a selection from one open Photoshop image into another.
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