Difference between the move assignment operator and move constructor?

For some time this has been confusing me. And I've not been able to find a satisfactory answer thus far. The question is simple. When does a move assignment operator get called, and when does a move constructor operator get called?

The code examples on cppreference.com yield the following interesting results:

The move assignment operator:

a2 = std::move(a1); // move-assignment from xvalue

The move constructor:

A a2 = std::move(a1); // move-construct from xvalue

So has it do to with which is implemented? And if so which is executed if both are implemented? And why is there the possibility of creating a move assignment operator overload at all, if it's identical anyway.