Buttons initiate app-specific actions, have customizable backgrounds, and can include a title or an icon. The system provides a number of predefined button styles for most use cases. You can also design fully custom buttons.
For implementation details, see UIButton.
Consider adding a border or a background only when necessary. By default, a system button has no border or background. In some content areas, however, a border or background is necessary to denote interactivity. In the Phone app, bordered number keys reinforce the traditional model of making a call, and the background of the Call button provides an eye-catching target that’s easy to hit.