"Tach time" on a mechanical tachometer, is the number which corresponds loosely to operating hours of the engine, because it has counted engine revolutions. As Peter says, they generally read accurately at about 2300 RPM, and often say on the instrument case.
"Hobbs" meters, are simply electric clocks. They run when the electricity is turned on. This can be by the master switch, an oil pressure switch on the engine, or in a few cases, by a weight on wheels switch. If you're using the Hobbs reading to log time, it would be wise to know which of these hookups is used in the plane you're flying...