what is the semantic of a ref to a function or functor

I am reading a piece of sample code from boost.thread, which is copied
below in full:
// Copyright (C) 2001-2003
// William E. Kempf

#include <boost/thread/thread.hpp>
#include <boost/ref.hpp>
#include <iostream>

class factorial
{
public:
     factorial(int x) : x(x), res(0) { }
     void operator()() { res = calculate(x); }
     int result() const { return res; }

private:
     int calculate(int x) { return x <= 1 ? 1 : x * calculate(x-1); }

private:
     int x;
     int res;
};

int main()
{
     factorial f(10);
     boost::thread thrd(boost::ref(f));// boost::thread thrd(f) would
result in f.result()==0
     thrd.join();
     std::cout << "10! = " << f.result() << std::endl;
}

I happen to find that if I remove boost::ref from boost::thread
thrd(boost::ref(f)); the result would result in f.result() changing
from a correct value(3628800) to 0.
While I tried to understand why, I realized I never really learned what
a reference to a function or functor is. In the semantics of pure
function pointers, there's no difference between passing f or &f as
a parameter. I know references are not pointers. however, can any one
please kindly explain
the difference in this case? Thanks.

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