Objective-C is a primary language used to write Mac softwares (Mac OS softwares, iPhone/iPad applications etc) If you’re comfortable with the basic Object Oriented concepts and the C language, Objective-C will make a lot of sense.
XCode is a development tool used for programming in Objective-C and to design interfaces Interface Builder is used. (XCode is a free software and it works only in Mac . 🙂 )
You can develop iPhone/iPad applications, Mac OS applications etc. But you must have Mac OS to start development. So You can run Mac on your iBook or via virtaul machine.