Objective C has a rather different syntax to C++, and a more dynamic object model (you can add methods to existing classes, query the type of an object at runtime, etc.) It's basically an attempt to graft a Smalltalk-type object system on top of C, and is therefore is not quite as efficient as C++. It's in some ways a bit more pleasent to work with, but it lacks some important features (e.g. exceptions).
I wouldn't really recommend it over C++, as there aren't many libraries available for it (with the big exception being Cocoa/GNUStep, of course).