What is the difference in benefits between a narrow-stance push-up and a wide-stance push-up?

I'm not sure what's the term used for the two styles of push-ups,
but the narrow-stance is the one where your elbows are facing backwards.
While for the wide-stance, the elbows are facing outwards, which is I guess is the more common method.

What muscle group does the two different styles work more, and which is the more effective in chest muscle training?

I'm trying to increase my chest muscle size, and especially the inner chest muscles (where the left and right chest muscles meet in the middle).

Help is much appreciated.

Where can I buy this? Looks really useful.