boost::thread объясните нубу

#include <boost/thread/thread.hpp>
#include <iostream>
#include <windows.h>
using namespace std; 
 
void hello_world() 
{
  cout << "thread!" << endl;
  Sleep(5000);
}
void hello()
{
    cout << "hello" << endl;
}
 
int main() 
{
  boost::thread my_thread(&hello_world);
  boost::thread my_thread2(&hello);
  my_thread.join();
  my_thread2.join();
  boost::thread my_thread3(&hello);
 getchar();
  return 0;
}

#include <boost/thread/thread.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <iostream>
#include <windows.h>
using namespace std;
 
void hello_world(boost::mutex& mutex) 
{
  {
  boost::lock_guard<boost::mutex> lock(mutex);
  cout << "thread!" << endl;
  }
  boost::this_thread::sleep(boost::posix_time::seconds(5));
}
 
void hello(boost::mutex& mutex)
{
    boost::lock_guard<boost::mutex> lock(mutex);
    cout << "hello" << endl;
}
 
int main() 
{
  boost::mutex io_mutex;
  boost::thread my_thread(boost::bind(&hello_world, boost::ref(io_mutex)));
  boost::thread my_thread2(boost::bind(&hello, boost::ref(io_mutex)));
  my_thread.join();
  my_thread2.join();
  boost::thread my_thread3(boost::bind(&hello, boost::ref(io_mutex)));
  my_thread3.join();
  getchar();
  return 0;
}