When you build and run with Simulator as the destination, you run in the iOS Simulator application. There’s little to say about running in the Simulator, because it’s so intuitive. The Simulator window represents a device, and you can interact with it in some of the same basic ways as you would a device. Using the mouse, you can press the Home button and tap on the device’s screen; hold Option to make the mouse represent two fingers and Option-Shift to move those fingers in parallel. Menu items let you perform hardware gestures such as rotating the device, shaking it, and locking its screen; you can also test your app by simulating certain rare events, such as a low-memory situation.
What hardware and system the Simulator simulates depends upon your choices in Hardware → Device and Hardware → Version. If your app runs on either iPhone or iPad, you can choose which device is simulated as you choose your destination. The iPhone 4 device, Hardware → Device → iPhone (Retina), is displayed at double size, so that each pixel of the Retina display corresponds to a pixel of your computer’s monitor. The iPad device can be displayed at half or full size (choose from Window → Scale).