Celsius (or degrees Celsius) is a scale to measure temperature and is derived from an SI unit with the symbol °C.
Celsius is named after Anders Celsius and is a scale derived directly from the Kelvin scale and was established in 1742. Otherwise known as the centigrade scale, it is recognised and used worldwide.
The Celsius scale is exactly 273.15 below the Kelvin scale which is the SI unit for temperature.
Water melts just below 0 °C and boils just below 100 °C (assuming normal conditions). The Celsius scale is also commonly used to measure the weather (or atmospheric temperature) and body temperature to diagnose fever. A typical comfortable day is anywhere from 10 °C - 30 °C and a healthy body temperature is anywhere between 36.1 °C and 37.2 °C.
Degrees Celcius is measured using a thermometre.
Kelvin is the SI base unit for temperature and is has the symbol K.
Kelvin is named after 1st Baron Kelvin, William Thomson who was born in Belfast and studied engineering and physics at Glasgow University.
The scale was based on Lord Kelvin's theory whereby a) there was a need for "absolute zero" or "infinite cold" to be represented by 0 and b) used degree Celsius to increment each unit.
The Kelvin has also been adapted to measure colour temperature and noise temperature; both with the capability to express amounts of "warmth". The photosphere of the sun, for example, has an effective colour temperature of 5778K.