Celsius is the standard unit of temperature in the metric system of units. Celsius is split up into degrees, with one degree being 1/100 of the temperature difference between the boiling and freezing points of water. A change by 1 degree Celsius is equal to a change of 1 Kelvin, however the Kelvin scale is offset by +273.15 degrees (0°C = 273.15 K).
The Celsius scale was created by Anders Celsius, who was a Swedish astronomer. Originally, Anders set 0°C to be the boiling point of water, and 100°C to be the freezing point! It is likely that the scale was reversed to how it currently exists (0°C as the freezing point) by Daniel Ekström, a manufacturer of scientific instruments.
The Celsius scale is occasionally referred to as Centigrade, because there are 100 degrees in the scale between the boiling point of water, and its freezing point.