I use a variety of hard drives from different manufacturers and the SMART data labelling seems to change between manufactures. Some call it power cycles and some the number of power on's or even boot or load count
When talking about MTBF "Mean time between failures" for normal mechanical hard drives there are a few areas that you should look at.
Power on hours or flying hours - This is the number of hours the drive has been in active use for.
Number of power on's or boots / loads - number of times from a complete stop of the drive and then started again. I have tested with my WD green drive in my current machine and it does not increase with a reboot as the drive continues to spin but if i power the pc off for a moment and then back on the number increments by one as you'd expect.
Number of bad sectors or unrecoverable errors - this could be indication that your drive is about to fail. Most motherboards check this value at boot time and will report to the user a warning if the number falls outside of normal operation specification.
All three of the above SMART data values will give you an idea of the mileage and remaining life of your drive. All drive manufacturers will provide the MTBF information in the drives technical specification, check out their website.
I'll continue to test with my other drives and report back in a comment but currently I believe only a full power off and boot again would count as a start or load count.
If you have spin down enabled on your OS this would also count as the drive motor has been stopped and started again.
Most consumer drives are designed to last between 3 to 5 years depending on their use and operating conditions.
Hard worked or hotter drives tend to fail far sooner than the advertised MTBF.
WD Red drives have been proven to last longer in a busy NAS environment.
With SSD drives they messure the drives life in read/write counts.
Here is a really good resource that explains it in detail:
http://www.pcmag.com/article2/0,2817,2404260,00.asp