What's the difference between assignment operator and copy constructor?

I don't understand the difference between assignment constructor and copy constructor in C++. It is like this:

class A {
public:
    A() {
        cout << "A::A()" << endl;
    }
};

// The copy constructor
A a = b;

// The assignment constructor
A c;
c = a;

// Is it right?

I want to know how to allocate memory of the assignment constructor and copy constructor?