I'm sure there is a wiki...
Short answer is back in the day when Intel had the hot P4, they came up with the BTX form factor. Electrically it's identical to ATX. But parts were flipped around so that it could be cooled better. Remember that heat rises, so some ATX things like GPUs aren't good for thermals. But then Intel gave us the C2D and didn't need BTX, so it died. OEMs were really the only ones to use it. Not very common these days.