Too complicated to explain here. A YouTube channel that will
give you thorough, well thought out, and occasionally funny
tutorials on the subject however, is
Although iPads can download apps made only for iPhone/iPod Touch, the iPhone can only download iPhone or iPhone/iPad apps. Apps made only for the iPad do not appear on the App Store. Luckily most of these apps have an iPhone counterpart such as Angry Birds (iPhone) and Angry Birds HD (iPad only).