Objective C is a language, being used for development of apps for Apple mobile platforms running iOS, such as iPhone or iPad
Objective-C - general purpose, object-oriented programming language used by the Apple Corporation. Objective-C is built on the C language with the added paradigms of Smalltalk.
In other words:
Objective-C - the programming language used to create applications for devices running on the operating systems OS X and iOS, such as, the iPhone and the iPad.
What is it used for?
If you are developing a Web portal or a website, you can sometimes create a version of the site, which will be properly displayed on mobile devices. However, in some cases, it is logical to develop a separate application for a mobile platform, using Objective-C, in order to attract new (or keep current) users.