oc

Short list of some of the major differences:

C++ allows multiple inheritance, Objective-C doesn't.
一个允许多继承，一个不允许
Unlike C++, Objective-C allows method parameters to be named and the method signature includes only the names and types of the parameters and return type (see bbum's and Chuck's comments below). In comparison, a C++ member function signature contains the function name as well as just the types of the parameters/return (without their names).
OC允许参数命名，方法的signature包含了名称，包括参数类型，返回类型。
而c++的成员函数signature包含函数名，和参数类型，返回类型，不包括名字
C++ uses bool, true and false, Objective-C uses BOOL, YES and NO.
bool的不同
C++ uses void* and nullptr, Objective-C prefers id and nil.
void*的不同
Objective-C uses "selectors" (which have type SEL) as an approximate equivalent to function pointers.
？？
Objective-C uses a messaging paradigm (a la Smalltalk) where you can send "messages" to objects through methods/selectors.
messaging机制
Objective-C will happily let you send a message to nil, unlike C++ which will crash if you try to call a member function of nullptr
可以发送消息给nil
Objective-C allows for dynamic dispatch, allowing the class responding to a message to be determined at runtime, unlike C++ where the object a method is invoked upon must be known at compile time (see wilhelmtell's comment below). This is related to the previous point.
oc允许dynamic dispatch，允许class运行时对消息进行反映。而c++的方法必须在编译时确定
Objective-C allows autogeneration of accessors for member variables using "properties".
支持properties，实现队成员变量accessors的自动生成
Objective-C allows assigning to self, and allows class initialisers (similar to constructors) to return a completely different class if desired. Contrast to C++, where if you create a new instance of a class (either implicitly on the stack, or explicitly through new) it is guaranteed to be of the type you originally specified.
Similarly, in Objective-C other classes may also dynamically alter a target class at runtime to intercept method calls.
Objective-C lacks the namespace feature of C++.
Objective-C lacks an equivalent to C++ references.
Objective-C lacks templates, preferring (for example) to instead allow weak typing in containers.
Objective-C doesn't allow implicit method overloading, but C++ does. That is, in C++ int foo (void) and int foo (int) define an implicit overload of the method foo, but to achieve the same in Objective-C requires the explicit overloads - (int) foo and - (int) foo:(int) intParam. This is due to Objective-C's named parameters being functionally equivalent to C++'s name mangling.
Objective-C will happily allow a method and a variable to share the same name, unlike C++ which will typically have fits. I imagine this is something to do with Objective-C using selectors instead of function pointers, and thus method names not actually having a "value".
Objective-C doesn't allow objects to be created on the stack - all objects must be allocated from the heap (either explicitly with an alloc message, or implicitly in an appropriate factory method).
Like C++, Objective-C has both structs and classes. However, where in C++ they are treated as almost exactly the same, in Objective-C they are treated wildly differently - you can create structs on the stack, for instance.

In my opinion, probably the biggest difference is the syntax. You can achieve essentially the same things in either language, but in my opinion the C++ syntax is simpler while some of Objective-C's features make certain tasks (such as GUI design) easier thanks to dynamic dispatch.
我觉得最大的不同是语法，俺觉得C++ syntax的语法更简单。而oc对dynamic dispatch的支持更好

Probably plenty of other things too that I've missed, I'll update with any other things I think of. Other than that, can highly recommend the guide LiraNuna pointed you to. Incidentally, another site of interest might be this.

I should also point out that I'm just starting learning Objective-C myself, and as such a lot of the above may not quite be correct or complete - I apologise if that's the case, and welcome suggestions for improvement.

EDIT: updated to address the points raised in the following comments, added a few more items to the list.

While they are both rooted in C, they are two completely different languages.

A major difference is that Objective-C is focused on runtime-decisions for dispatching and heavily depends on its runtime library to handle inheritance and polymorphism, while in C++ the focus usually lies on static, compile time, decisions.
主要的不同是oc关注运行时的决策，用于dispatching，严重依赖于运行时库来处理inheritance and polymorphism。而c++关注静态，编译时的决策
Regarding libraries, you can use plain C libraries in both languages - but their native libraries are completely different.

Of interest though is that you can mix both languages (with some limitations). The result is called Objective-C++.

They're completely different. Objective C has more in common with Smalltalk than with C++ (well, except for the syntax, really).
oc和smalltalk更相似，而不是c++