in pass by referance the parameters are passed as the
address of the variables whereas in pass by value the
variables are directly passed as parameters

In case of pass by value..the change in the sub-function 
will not cause any change in the main function wheareas in 
pass by reference..the change in the sub-function will 
change the value in the main function..

passing an object by value is defined by the copy 
constructor of that object's class. This can make pass-by-
value an extremely expensive operation.

in pass by referce function can access original memory 
location of variable so can access value while in case of 
pass by value fuction can copy the value of variable to the 
prototype of its kind of variable so calling funcion can 
access the prototype variable but not the direct original 
variable.

in Pass by value; if any change in variable in the sub-
function may not reflected to the main function. where as 
in pass by reference the change in the variable may reflect 
to the original value in the main funtion.

ex : //  Pass by Reference

void Get( int &nIndex){
  nIndex = 10;
}

void main()
{
  int x = 100;
  cout<<Get( x );
}

o/p : 10;

ex : //  Pass by Value

void Get( int nIndex){
  nIndex = 10;
}

void main()
{
  int x = 999;
  cout<<Get( x );
}

o/p : 999

Pass By Value:

1.It passes the actual value of parameter from the function 
call.
2.Value of the parameter variable in calling functions 
remains unchanged.
3.copy of the variable of actual parameter are made and 
transfer to the formal parameter.

call By Reference:

1.the address of the actual parameter has to be transfered
2.the formal parameter in the function decleretion must of 
pointer type.
3.Any changes in the formal parameter can change the value 
of  actual patameter.