The operating system of iphone is known as iOS. The applications based on iOS are developed exclusively for the Apple Inc. products. The iOS applications are basically developed using the languages as Swift, Objective C and Xcode.
Swift is a multi paradigm, general purpose, compiled programming language used for iOS. It is designed to work with Apple's Cocoa and Cocoa Touch frameworks and the large body of existing Objective-C code written for Apple products.
Objective-C is also a multipurpose but Object oriented programming language. It adds Smalltalk-style messaging to the C programming language. Objective-C was the main programming language for iOS and its application programming interface before the introduction of Swift. Xcode used for the development of iPhone app development is the integrated Development Environment (IDE) for the macOS. It has a suite of software development tools required for the app development in iOS platform. Xcode 9 is the latest xcode series. It was publicly released in September 2017.
The development Kit required for the creation of iphone applications are is iOS SDK (Software Development Kit). It is free for download to the Mac Users and it is not available for Microsoft users. iOS Software Development Kit is developed by Apple Inc. At first the company was reluctant to introduce third party application for the development of the iOS application now iOS SDK combined with Xcode helps the developers in writing the applications by using other officially supported languages as Swift that was introduced at Apple's 2014 Worldwide Developers Conference (WWDC).