As an aircraft renter, all I ever knew was Hobbs time. It was what I logged with and it was what I was billed with. For every 0.1 hour the engine turned, I paid $4.90 (this was almost 20 years ago) and logged 0.1 hours in my book. It was simple and predictable. FBOs and flight schools prefer to bill for aircraft usage on Hobbs time because it is an accurate indicator of how long the aircraft is not available to them and other customers.
When 
I became a fractional owner of a Piper Cherokee 140, my attention shifted to Tach Time. The club's cost recovery was based on tachometer time, maintenance was done on tach time, but I still logged Hobbs time. Tach time is a better representation of how the aircraft is used. Tach time ticks ahead more quickly the faster the engine is turning. The idea being that if you are taxiing at 1000 rpm, you are only working the engine 40% as hard as it would be if you were flying, and thus you will end up at that 50 hour oil change or 100 hour inspection at a slower pace.
These two methods of time keeping have resulted in many hours of discussion among pilots and aircraft owners. I certainly have been a part of many of them. But did you know there is a third? It's called Time in Service or Air Time. It is defined as follows:
Time in service, with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.
Time in service plays a critical role in aircraft management and maintenance. For example, 100-hour inspections are to be completed every 100 hours of time in service. Not 100 hours on the Hobbs. Not 100 hours on the tachometer.
(b) Except as provided in paragraph (c) of this section, no person may operate an aircraft carrying any person (other than a crewmember) for hire, and no person may give flight instruction for hire in an aircraft which that person provides, unless within the preceding 100 hours of time in service the aircraft has received an annual or 100-hour inspection and been approved for return to service in accordance with part 43 of this chapter or has received an inspection for the issuance of an airworthiness certificate in accordance with part 21 of this chapter. The 100-hour limitation may be exceeded by not more than 10 hours while en route to reach a place where the inspection can be done. The excess time used to reach a place where the inspection can be done must be included in computing the next 100 hours of time in service.
Now, this does not mean all of us who have been maintaining aircraft based on hobbs or tach time are in violation of the regulations. But what it does mean is many of us may have been over-maintaining our aircraft as you will typically reach the 100-hour inspection more quickly.
