Mock objects Vs Real objects

• In Unit testing, why do we use mock objects?

• How are mock objects better than real objects in unit testing?

• When you create a mock object, what exactly it means? For example let's say you create

  A a = Mockito.mock(A.class);

  Does it create an object of class A? or something else?

• When you create a mocked object and use it in calling a method, say a.method1(), how does the mocked object knows that it should call method1 itself?

Kindly let me know about these.