The 'V' denotes the arrangement of the cylinders which are, facing the front of the engine, in a 'V' configuration. Two on the right and two on the left (or three if it's a V6).

A straight (in line) four, six or (even eight) will have the cylinders in a line.

The difference between the two is that, all things being equal, the V6 will (probably) be of a larger capacity, more powerful....and considerably more fuel hungry.

Making sure your engine goes...