Alpha channels are used in graphics and video editing/compositing applications. An alpha channel is a special channel in the image used for transparency. It can be thought of as a mask, matte or stencil. Its purpose is to define the areas of an image which can be interpreted as transparent, should the user wish to make use of the image's transparency.
Alpha channels are supported by most well-known applications including Adobe, Apple, etc.
32-bit graphics systems contain four 8-bit channels: one each for red, green and blue, plus one alpha channel. Some applications use a 16-bit alpha channel.