Cocoa Touch is the application-development environment for iOS, the touch-device counterpart to the Cocoa environment that Apple uses on the Mac. Apple’s developer documentation states that “Cocoa and Cocoa Touch are the application development environments for OS X and iOS, respectively,” and that “Cocoa Touch, which includes Foundation and UIKit frameworks, is used for developing applications that run on iOS.” Both environments include the Objective-C runtime and a pair of core frameworks.
The two core frameworks divide the work. Foundation “defines the basic behavior of objects,” providing the NSObject root class, primitive value types such as strings and numbers, collections like arrays and dictionaries, and operating-system services including file management and internationalization. UIKit sits on top, supplying the objects an application shows on screen and the structure for application behavior, including event handling, drawing, and UI elements such as table views, sliders, buttons, text fields, and alert dialogs.
Cocoa Touch is an adaptation rather than a straight copy of desktop Cocoa. Apple’s docs note that although the iOS infrastructure on which Cocoa Touch depends is similar to that for Cocoa on the Mac, there are significant differences. The most visible is input: where the Mac’s AppKit framework is built around a mouse and keyboard, UIKit is built around multi-touch gestures, accelerometer and other sensor input, and the constraints of small, battery-powered screens.
The environment arrived with the iPhone and was opened to outside developers through the iPhone SDK in 2008. By packaging the proven Cocoa design patterns of model-view-controller, delegation, and target-action into a touch-first API, Cocoa Touch gave the new iOS platform a mature programming model from day one and let developers familiar with Mac development carry much of their knowledge across.
Cocoa Touch is best understood as the umbrella name for the iOS app layer rather than a single product: it is the combination of UIKit, Foundation, and the supporting frameworks (graphics, audio, persistence such as Core Data, and more) that an iOS app builds on. As Apple’s platforms and tools evolved, including the later arrival of Swift and SwiftUI, that underlying Cocoa Touch foundation continued to anchor native iOS development.