I am new to iOS development. I am trying to figure out these things.
waiting for answer Thanks :)
Personally, I find that the only real difference between the iPhone and the iPad is the UI and UX. Most of the code is the same for both applications. (In fact, you can even reuse a lot of code if you are doing a universal app!)
The main difference really is the screen size. The iPad is huge in comparison to the iPhone, and this difference gives you a lot more options to work with.
Another thing to note is that the iPad is usually done in landscape orientation, whereas the iPhone is done in portrait. This is not an absolute rule however, it's just the way I see it.
The graphics aren't too different between the devices, except that because of the large screen size, you can do a bit more detailed work with the iPad images then the iPhone images. (Even with the retina display, too fine detail on the iPhone is barely noticeable.)
The other thing I wanted to mention is the flow of the devices. Again, because of the screen size of the iPad, you can fit a lot more content on the screen, and therefore have to push to new views less frequently. I've worked on apps where we had 3 UIViewController's for the iPhone version, and only 1 for the iPad version.
In short, it depends really on your type of app. If you have a lot of content, and want to display it all at once, go for the iPad. If you want a streamlined, minimal approach to your app, go for the iPhone.
Again, this is all just my personal opinion. Hope that Helps!