What is the difference between these two equivalence models?

Question

Hi.I'm beginner on programming.Suppose we have defined this class.

class ClassA
{
//data
};

In this I defined copy constructor.
What's the difference between these two:

1-
classA c1;
classA c2;
c1 = c2;

2-
classA c1;
classA c2 = c1;

Why copy constructor isn't called in the first but is called in the second????