Celsius refers to a scale devised by Swedish astronomer Anders Celsius (1701 - 1744) but for two centuries it was referred to as the centigrade scale. Worldwide, the Celsius scale is the most common way of reporting temperatures except for one country.

Gabriel Fahrenheit (1686-1736) devised this scale which now is rarely used except in the United States

The Kelvin scale is named for Baron Kelvin (William Thomson) (1824 - 1907) and has zero degrees defined as the temperature of absolute zero and its increments of temperature are exactly the same as the Celsius scale. For example, 273.15 K to 323.15 K precisely equals the 50 degrees of the 0 to 50 degrees of the Celsius scale.

The Rankine scale, named for William Rankine (1820 - 1872) also defines the absolute zero temperature as 0 degrees. This scale works similarly to the Kelvin except that its increments are based on the Fahrenheit scale. For example, 459.67 to 509.67 on the Rankine scale precisely match the 0 to 50 degrees of the Fahrenheit scale.

The Reaumur scale proposed by Rene Reaumur (1683 - 1757) like the Celsius scale has 0 degrees as the melting point of water but the boiling point of water is 80 degrees. So basically, the increments of the Reaumur scale are four fifths the size of the Celsius scale. Therefore, 20 to 24 degrees Reaumur exactly equal 25 to 30 degrees Celsius.
This temperature scale held high popularity in Europe and Russia for over two centuries but by the late twentieth century its use had been superseded by the Celsius scale and the use of the Reaumur temperature scale has become exceedingly rare.