To confuse things further, you can mix Objective-C and C++ (this is generally referred to as Objective-C++).
We have had many (sometimes heated) discussions about Objective-C and C++ here, the most prolific of which is here:
https://forums.macrumors.com/threads/246143/
I won't have too much to add to what people have already stated, but at the basest level the languages are related only in-so-far as they were based (to different degrees) on C. If you look at a routine dealing with primitives (and structs, which I would consider primitive) in C++ and Objective-C they will look similar, but if you look at something dealing with either language's object models, they will look totally different (although now, with the ability to access properties in Objective-C 2.0 using dot notation, that would look a little more similar).
I guess to speak in broad terms, the main differences are the object models. Most of the major differences come from how objects are defined, how their properties are accessed, how their behaviors/functions/methods are invoked, inheritance models, abstract objects vs. protocols, etc. Other "smaller" differences like operator precedence are really differences in C and C++, and that difference was just inherited from C by Objective-C.
-Lee