Objective-C is Swift's predecessor, and has always been the primary language when it comes to iOS or OS X applications. It is a general purpose object-oriented programming language that builds on the C programming language. Objective-C, unlike C++, is a superset of C, which means that any correct C program is also a correct Objective-C program. Like C++, Objective-C adds the capability of object-oriented programming by adding classes. Nowadays, new applications are mainly written in Swift, because this language is a lot more modern, but Objective-C is still officially supported by Apple.
What does a Objective-C developer do?