Source : Free On-Line Dictionary of Computing

bits per pixel
     
         (bpp) The number of {bits} of information
        stored per {pixel} of an {image} or displayed by a {graphics
        adapter}.  The more bits there are, the more colours can be
        represented, but the more memory is required to store or
        display the image.
     
        A colour can be described by the intensities of red, green and
        blue ({RGB}) components.  Allowing 8 {bits} (1 {byte}) per
        component (24 bits per pixel) gives 256 levels for each
        component and over 16 million different colours - more than
        the human eye can distinguish.  {Microsoft Windows} [and
        others?] calls this {truecolour}.  An image of 1024x768 with
        24 bpp requires over 2 MB of memory.
     
        "High colour" uses 16 bpp (or 15 bpp), 5 bits for blue, 5 bits
        for red and 6 bits for green.  This reduced colour precision
        gives a slight loss of image quality at a 1/3 saving on
        memory.
     
        Standard {VGA} uses a {palette} of 16 colours (4 bpp), each
        colour in the palette is 24 bit.  Standard {SVGA} uses a
        {palette} of 256 colours (8 bpp).
     
        Some graphics hardware and software support 32-bit colour
        depths, including an 8-bit "{alpha channel}" for transparency
        effects.
     
        (1999-08-01)