W12 and V12
well the V12 is a block with to banks 6 cylinders on each bank w12 has 3 banks in a "w" position with 4 cylinders to each bank
The best way to discribe it would be that in a V12, the 6 cylinders on each side or directly in line - - - - - - , where as in a W12, there are still 6 cylinders per side, but instead of being directly in line with each other they are off-set -_-_-_ and still share a common crankshaft. the a 6.0 liter W12 with be shorter front to back than a 6.0 liter V12 thus allowing for less space taken by the engine.
If you're referring to the W12 made by Volkswagen and on Audi and VW products, it's a 4 bank engine that is practically two narrow bank V6's put together Literally it's two VR6 engines which share a common crank shaft (with obvious alterations but geometry wise they are nearly identical or were originally...). When VW announced their W12 they also announce a W8 and have since created a W16 (In the Veyron). If I remember the W12 is a few inches shorter than a small block Chevy V8, but taller and wider.
it should be noted that the W12 is not a true '4 bank' engine as their is only one cylinder head for each set of 6 psitons.