When we use software, we expect it to act a certain way based on convention. When we see something that looks like a button, we expect to be able to tap it, and for some event to happen when it is tapped. Part of this comes from the fact that some methods of interaction are universally intuitive, and have been established for a long time. However, most application development environments come with a set of Human Interface Guidelines (HIG), which outline the intended look, feel, and use of the software being created.
Apple, famous for its strict policies over design, has a very thorough set of HIG available for developers that make it easy to understand how they expect your software to function. While the full set of documentation covers many aspects of app interactions, we're going to focus on the standard gestures and what users expect from them.