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When you look up close at iPhone native and Web applications and compare their functionality, you will begin to see design patterns in how information is presented to a user. A list is presented this way. A page of menu options is presented that way. Some apps may break the rules, but by and large, applications follow common "look and feel" guidelines because they know users expect and rely on a consistent and intuitive way to perform a task.
To achieve that consistency and to avoid reinventing the wheel, developers often turn to application frameworks. In the generic sense, an application framework is a toolkit that you can use to implement a standard structure for an application for a given platform. Windows programmers utilize the Windows Software Development Kit (SDK) for creating apps for that desktop platform. So, too, developers writing native iPhone apps use the iPhone SDK.