I cannot tell you how many people I’ve met that calls everything a V4, V6, V8, V10, V12. That really annoys me.
V Engines
This is a V8 engine. Why? Because it’s cylinders form a V.
Flat Engines
Now this is a flat engine because it’s flat. Get it?
Inline/Straight Engines
An inlne/straight engine is when all the cylinders are in a line point straight up, or slanted to the side, as in a slant engine.
W Engines
Now this one is a lot harder. Picture two V6’s, side by side. Good, that’s a W12. How about two V8’s? Alright, W16. And so on and so forth.
Now, there’s other types too, such as rotary, but seeing that it doesn’t have cylinders, people can’t call it a V4/V6/V8/V10/V12.
Sources: http://www.h1v8.com/, http://www.sherline.com, http://www.bimmerforums.com/, http://en.wikipedia.org/
Automotivation 
