Look up: Alpha-Blending

1. 
   Alpha Blending
   The real world is composed of transparent, translucent, and opaque objects. Alpha blending is a technique for adding transparency information for translucent objects. It is implemented by rendering polygons through a stipple mask whose on-off density is proportional to the transparency of the object. The resultant color of a pixel is a combination of the foreground and background color.
   Found on http://www.directron.com/videoglossary.h
   
   
2. 
   Alpha-Blending
   Alpha-blending is a technique which can be used when computer graphics are laid on top of each other and one or more of the objects contain a transparent, or semi-transparent, portion. It ensures that the pixels of the graphic which are underneath a transparent area are visible through it and that their colour or brightness is adjusted according to the degree of transparency of the upper object. The alpha channel is really a form of mask, dictating what amount of information should be allowed to show through from lower-lying graphics.
   Found on http://www.meko.co.uk/alphablend.shtml