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to create copies of the object

class person{
String name;
int age;

public person(){
}

public person(person p)
name=p.name;
age=p.age;

}

person(person) is copy consructor

it's C++. In C++ copy constructor often used inside stl. For example when stl::vector needs to resize it allocates new memory block and calls copy constructor for each object in container to perform copy from old to new memory block.

1) when an object passed by value to a function
2) when an object is returned by value from a function
3) in cases like:
Sample object2 = object1;
Sample object2(object1);

no... the cases 1 and 2 operatoroverloading of ==.does the job of assigning.
copy constructor is called when an object is created and its parameter is an object of same type

sorry operatoroverloading of = instead of ==

From wikipedia,

The following cases may result in a call to a copy constructor:

1. When an object is returned by value
2. When an object is passed (to a function) by value as an argument
3. When an object is thrown
4. When an object is caught
5. When an object is placed in a brace-enclosed initializer list

Also when you are creating a new object from an existing object.

Copy constructor is called when:
1) An object is passed to a function by value
2) Object is returned from a function by value
3) Create a new object from an existing object
4) In containers like vector, when the re-sizing of vector happens.