When somebody says it has a V12 6 cylinder 3 litre engine, what does the V12 mean and stand for?

 

12 cylinders in a V-pattern layout and 3L is the displacement. Most foreign manufacturers use liters to measure displacement, we use cubic inches here in the states so that's when you hear "its a 265cu. in. V8" that translates to 4.3L.
There are V-pattern layouts, inline layouts such as the BMW inline six cylinder engines. Porsche and Subaru use "H" or Boxer pattern engines in four and six cylinder variants.

 

Displacement (like 3 liters, or 1200 cubic centimeters, or 350 cubic inches) is the volume displaced by the pistons in one crankshaft revolution (all the pistons go up and own once). So for instance, an engine with a 4" diameter piston, 8 cylinders, and a 3" stroke will have displacement of 8 * pi-r- squared (r is 2) * 3. The result is 302 cubic inches.

Vee shaped engine blocks are about half as long as an inline engine with the same dimensions. This mostly helps with fitting it in the car or motorcycle or airplane. Structurally it improves certain things about the engine itself. For instance, the crank is shorter, so it deflects a lot less.

 

V12 or 6 cyl. Can't be both.

 

The V simply refers to the positioning of the cylinders. The number shows how many cylinders the engine has. That means a V12 engine has 12 cylinders.

 

I suspect the OP misunderstood something, or there is a typographical error involved. I bet he meant to say 12V (12 valve) instead of V12. A 12V 6 cylinder engine has 2 valves per cylinder, a common arrangement.