There are three distinct approaches to iOS development

Web Application Development

The original iPhone OS 1.0 required all non-Apple applications to be web-based and executed within the Mobile Safari web browser. Because Mobile Safari does not support plugins like Adobe Flash or Microsoft Silverlight, this meant that all third party applications were originally written in HTML, CSS, and JavaScript. This method of development remains a viable option today, especially for applications that must be accessible on a wide range of devices or for development teams with an aversion to Mac OS X and Objective-C.

Native Application Development
With the release of iPhone OS 2.0 and the introduction of the iPhone SDK and iTunes App Store, developers have been encouraged to write native applications for the iPhone using Objective-C and Xcode. Native applications are compiled binaries that are installed and executed on the user’s device. These applications are granted considerable access to the device hardware, and only native application binaries can be distributed through the iTunes App Store. Because the iPhone OS runs on iPod touch, iPhone, and iPad devices, most applications can be built for all three devices with only minor code variations, though there are significant advantages to optimizing your application for the much larger iPad screen.

Hybrid Application Development
It is also possible to combine the above approaches and create iPhone applications that are installed on a user’s device, are written primarily in HTML, CSS, and JavaScript, and are released through the iTunes App Store. Such applications are growing in popularity thanks to open-source libraries likeQuickConnect and platforms like PhoneGap, AppCelerator, and rhomobile.