An unsigned 32-bit value used by the Windows GDI to define a color. The first byte specifies the red channel, the second the green channel, the third the blue channel, and the fourth is unused and should be zero:
0x00bbggrr
More recent versions of Windows apparently support an alpha channel, but I haven't looked into that because most people still use older versions of Windows so you can't count on it being there.
See also RGBQUAD, which is also 32 bits long and does the same thing, but has a different byte order.