About Photoshop Masks

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.

Examples of masks
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 stored as an alpha channel, you can reload the selection at any time or even load it into another image.

Selection saved as an alpha channel in Channels panel

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.

About layer and vector masks

You can use masks to hide portions of a layer and reveal portions of the layers below. You can create two types of masks:

• Layer masks are resolution-dependent bitmap images that are edited with the painting or selection tools.
  
• Vector masks are resolution independent and are created with a pen or shape tool.
  
  
  Layer and vector masks are nondestructive, which means you can go back and re‑edit the masks later without losing the pixels they hide. Read more about nondestructive editing here.
  
  In the Layers panel, both the layer and vector masks appear as an additional thumbnail to the right of the layer thumbnail. For the layer mask, this thumbnail represents the grayscale channel that is created when you add the layer mask. The vector mask thumbnail represents a path that clips out the contents of the layer.
  
  
  Note: To create a layer or vector mask on the Background layer, first convert it to a regular layer (Layer > New > Layer from Background).
  

  Masking layer

  A. Layer mask thumbnail B. Vector mask thumbnail

  C. Vector Mask Link icon D. Add Mask

You can edit a layer mask to add or subtract from the masked region. A layer mask is a grayscale image, so areas you paint in black are hidden, areas you paint in white are visible, and areas you paint in shades of gray appear in various levels of transparency.

Basket layer mask: Background painted with black;
description card painted with gray;
basket painted with white

A vector mask creates a sharp-edged shape on a layer and is useful anytime you want to add a design element with clean, defined edges. After you create a layer with a vector mask, you can apply one or more layer styles to it, edit them if needed, and instantly have a usable button, panel, xr other web-design element. 

— From http://helpx.adobe.com/photoshop.html.
Read more about Layer Masks at http://helpx.adobe.com/content/help/en/photoshop/using/masking-layers.html and http://helpx.adobe.com/content/help/en/photoshop/using/masking-layers.html