Objective-C is an extension of the C language, developed around 1982-1983 by Brad Cox and Tom Love. Objective-C is fully compatible with C (a plain C program can be compiled with an Objective-C compiler), but Objective-C is also extending the C base language with a few constructs, such as classes, messages and inheritance.
Those Objective-C specific extensions are based upon Smalltalk, and in particular Smalltalk-76, because the object model used by Objective-C is the one used in Smalltalk-76 (with one particular class being the root class, superclass of both class and metaclass hierarchies). A separate metaclass hierarchy, as used in later Smalltalk-80 systems, has so far not yet been used in Objective-C.
Objective-C has over the years also become a popular user-interface development language, because, as an extension of C, it is sufficiently low-level for system programming tasks, yet at the same time the language supports the development of dynamic frameworks in the style of Smalltalk MVC (Model-View-Controller) libraries.