Depth is the amount of data that is stored for a single pixel. This can be 8, 15, 16 or 24 bit. (2^8=256, 2^24=16.7 million ..)
The other color variable is "bits per pixel" (bpp). This is the amount of memory on the card reserved for a pixel. This amount can be 8, 16 or 32. To store a 24bit pixel on 32 bits are used. (it is easy to work with powers of 2) A depth of 32bit would be 2^32 colors but that doesn't exist.
On windows "32bit" means 24bit (RGB) + 8bit alpha. This is not 2^32 colors.
