Temperature is a property of an object or substance that expresses numerically how hot or cold it is. Heat (or otherwise) is the magnitude of the presence of thermal energy; which is present in everything.
To measure temperature, you need a device sensitive to thermal energy; these are called thermometers and are designed to measure heat in various different ranges; this is called ‘device calibration’. A device calibrated to measure very cold temperatures would very sensitive to pick up the small readings; and would likely be damaged if used in a high range such as a furnace or in a volcano.
The lowest theoretical temperature is absolute zero. This represents a state where there exists no thermal energy whatsoever. The most common units of temperature used in the world are Celcius (previously centigrade) - °C, Farenheit – °F, Kelvin – K and Rankine – °R. Kelvin is the SI base unit – and is used predominantly for scientific purposes.
Different countries still use different units to express things like the atmospheric temperature to describe the weather or body temperature to diagnose fever. For exmaple; the UK largely uses Celcius (°C) and the USA uses Farenheit (°F) - this gives rise to the tools like the Celcius to Farenheit converter and the Farenheit to Celcius converter and makes them useful for people translating between the two.