Although initially defined by the freezing point of water (and later the melting point of ice), the Celsius scale is now officially a derived scale, defined in relation to the Kelvin temperature scale.
Zero on the Celsius scale (0°C) is now defined as the equivalent to 273.15K, with a temperature difference of 1 deg C equivalent to a difference of 1K, meaning the unit size in each scale is the same. This means that 100°C, previously defined as the boiling point of water, is now defined as the equivalent to 373.15K.
The Celsius scale is an interval system but not a ratio system, meaning it follows a relative scale but not an absolute scale. This can be seen because the temperature interval between 20°C and 30°C is the same as between 30°C and 40°C, but 40°C does not have twice the air heat energy of 20°C.
A temperature difference of 1 deg C is the equivalent of a temperature difference 1.8°F.