An iOS application is a compiled executable along with a set of supporting files in a bundle. The application bundle is packaged into an archive file to be installed onto a device or upload to the App Store.
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Xcode can be used to run iOS applications in a simulator, as well as testing them on a local device. Submitting an application to the App Store requires a developer signing key, which is included as part of the Apple Developer Program at https://developer.apple.com.
Most iOS applications to date have been written in Objective-C, a crossover between C and Smalltalk. With the advent of Swift, it is likely that many developers will move at least parts of their applications to Swift for performance and maintenance reasons.
Although Objective-C is likely to be around for a while, it is clear that Swift is the future of iOS development and probably OS X as well. Applications contain a number of different types of files, which are used both at compile time and...