There are a few main types of hour recording that you need to know about as the aircraft hirer (there may be other types or the method used to record them may vary depending on aircraft configuration).
1. VDO / Taco / RAW engine hours
Depends on aircraft configuration, however for some aircraft, starts to tick over when engine RPM exceeds threshold, for example, 1,500 rpm. As you can imagine, it will only tick over hours during runups, followed by takeoff and flight. Alternatively, some configurations start the hours ticking as soon as the engine is running. Engines with FADEC for example, start ticking over hours at 1,500rpm+.
2. Airswitch
Ticks over hours once the aircraft reaches a certain airspeed.
If you can hire an aircraft on airswitch time for a similar rate to VDO then you'll be laughing financially.
During circuits at an airport such as Bankstown or Moorabbin where it's often busy, the difference can be significant, at a rate of around 1.4.
1.0 hours airswitch.
1.4 hours taco / vdo.
Some owners feel that if the hirer is paying for the aircraft based on airswitch, then they are more likely to treat the aircraft well, for example, warm it up, don't over-rev it etc.
It is fair enough however, to be paying for VDO when an instructor in on board, otherwise expect the hourly rate to be higher for airswitch.
TTIS in the case of an advert for a used aircraft would normally be based on airswitch.
Unfortunately there is often debate in engineering / LAME circles where some feel that maintenance should not be performed on airswitch times, but rather VDO, as after all, the engine is still running for those hours. Compare to a car, oil change at 10,000 hours, but if a lot of those KMS are done in stop start or barely moving traffic, do you do the oil change at 14,000 kms instead?