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/