The Hobbs meter is an actual electric clock that runs when the engine is running, usually by sensing oil pressure. Most rental planes have a hobbs.

Tach time is basically a mechanical counter that is drive by engine RPM. It is usually calibrated to approximate actual clock time at the typical cruise RPM for the engine, often 2300 rpm. Maintenance is based on tach time, so every airplane should have one. Some rentals save the expense of installing a hobbs by just using tach instead of hobbs for billing.

The good thing about this is that while taxiing or flying slowly, you get charged for less time. If a club has their rates adjusted for a typical renter's high-speed cross-country, a time builder might be able to log more time and get charged less by flying slowly at reduced power settings and engine RPM. Pattern work usually results in lower tach times, IIRC.

Many folks commonly multiply tach x 1.3 to get loggable time, but you can always legally use your watch and log the actual time from engine start to engine stop as flight time regardless of what the tach says.