Difference between time_t and clock_t

The example:

#include <iostream>
using namespace std;

int main()
{
time_t time_start, time_end;
time(&time_start);
sleep(5);
time(&time_end);
cout<<difftime(time_end, time_start)<<endl;  // 5

clock_t clock_start, clock_end;
clock_start = clock();
sleep(5);
clock_end = clock();
cout<<(clock_end-clock_start)/CLOCKS_PER_SEC<<endl;  // 0
return 0;
}

It is clear to see that time_t is used to test the real time, while clock_t is used to test the cost of CPU time, because sleep(5) don't occupy any CPU resource.