Well, partially right.....
Lets break down the words. Thermometer is Thermo (Temperature), Meter (Measure). So all a Thermometer will do is measure the temperature...
Yet,
Thermostat is Thermo (Temperature, again) and Stat (Statistics or Level). And like a stat, you can alter the temperature. So you not only get the current reading, but also can alter that reading.
If the unit only gives the reading, then its a Thermometer, if you can read and alter, it's a Themostat.