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The difference is that copy by value means the method which is called may not modify the content received. OTOH, copy by reference means a reference is passed so that any operations performed affect the referent.

The code below is off the top of my head, so it may not compile... but it'll probably serve as a demonstration.
The language author has covered your question on his site - http://www.research.att.com/~bs/bs_faq2.html

#include <iostream>

void byValue( int n )
{
	std::cout << "1. In byValue:   " << n << std::endl;   // initial value

	n = 32;

	std::cout << "2. In byValue:   " << n << std::endl;   // updated value
}

void byReference( int& n )
{
	std::cout << "1. In byReference:   " << n << std::endl;   // initial value

	n = 32;

	std::cout << "2. In byReference:   " << n << std::endl;   // updated value
}


int main(int argc, char* argv[])
{
	int x = 5;

	std::cout << "Before byValue:  " << x << std::endl;   // initialize

	byValue( x );

	std::cout << "After byValue:   " << x << std::endl;   // no change

	std::cout << "Before byReference:  " << x << std::endl;   // initialize

	byReference( x );

	std::cout << "After byReference:   " << x << std::endl;   // yep, changed!


	return 0;
}