 <p>Under Windows 7 on present generation processors, this is a reliable high precision (nanosecond) timer inside the CPU (HPET).</p> <p>Under previous versions and on previous generations of processors, it is "something", which can mean pretty much <em>anything</em>. Most commonly, it is the value returned by the RDTSC instruction (or an equivalent, on non-x86), which may or may not be reliable and clock-independent. Note that RDTSC (originally, by definition, but not any more now) does not measure <em>time</em>, it measures <em>cycles</em>.</p> <p>On current-and-previous-generation CPUs, RDTSC is usually reliable and clock-independent (i.e. it is now <em>really</em> measuring time), on <em>pre</em>-previous generation, especially on mobile or some multi-cpu rigs it is not. The "timer" may accelerate and decelerate, and even be different on different CPUs, causing "time travel".</p> <p><strong>Edit:</strong> The <code>constant tsc</code> flag in cpuid(0x80000007) can be used to tell whether RDTSC is reliable or not (though this does not really solve the problem, because what to do if it isn't, if there is no alternative...).</p> <p>On yet older systems (like, 8-10 years old), some other timers may be used for QueryPerformanceCounter. Those may neither have high resolution at all, nor be terribly accurate.</p>