I could be wrong, but my understanding is:
ACU stands for army Combat Uniform.
Their current issue to most of the army is the pattern most people call ACU. The white and grey.
Some units are transferring into Multicam, I believe it is set to become the new "ACU"
Now, when people refer to an ACU cut of another camouflage (Multicam, Cadpad, whatever) They are referring to the way the camo is cut. where the pockets are, how it fits, where the zippers are .ect
In my experience, most people prefer the newer ACU cut to the older BDU cut.
This is the older BDU cut, notice the collar and the 4 big square pockets.
This is the ACU cut. Again, notice the difference in the collar. There are also only 2 chest pockets, and they are on handy angles!
There are a number of other differences (zippers vs. Buttons, velcro .ect) but you really have to see and try it to get the difference. Theres also some differences in the pants.
Sorry if thats not a perfect explanation, I'm sleepy..