Objective-C is a programming language based on C, with influences from Smalltalk. It is object-oriented, featuring message-passing between objects and reflective querying of an object to discover its capabilities. It has no standard library (unlike C), but uses various system-specific frameworks (such as Apple's Cocoa).
Objective-C was developed in the early 1980s at a company called Stepstone, and adopted in the late part of that decade by NeXT (founded by Steve Jobs when he left Apple) as the main language used for the NeXTstep operating system. When Jobs returned to Apple and merged NeXT into that company, it became a major language for Mac OS X development, and eventually the official language for iOS development as well, until Swift was released in 2014 as a new iOS/OS X language.
C++ and C# take different approaches to object-oriented C. Objective-C's syntax uses square brackets to invoke a method of a class, a syntax not found in those other languages, though the more widely-used "dot" syntax of following an object name with an attribute is also used.