The Fahrenheit scale was proposed by Daniel Gabriel Fahrenheit in 1724. He set 0°F as the temperature of a brine mixture (water, ice, and ammonium chloride) and 96°F as an approximation of human body temperature — chosen so the scale between those points would divide into 12 equal parts. On this scale, water freezes at 32°F and boils at 212°F at standard atmospheric pressure, a span of 180 degrees.
The Celsius scale (originally Centigrade) was proposed by Anders Celsius in 1742 and later inverted by Carl Linnaeus to its current form. It defines 0°C as the freezing point of water and 100°C as the boiling point, both at standard pressure. Its 100-degree span between these two reference points is why it was called Centigrade (meaning 100 steps). The conversion between scales: °C = (°F − 32) × 5/9, and °F = (°C × 9/5) + 32.
Kelvin: the absolute temperature scale
The Kelvin scale is defined by absolute zero — the theoretical temperature at which all thermal motion ceases. At absolute zero (0 K = −273.15°C = −459.67°F), a system has minimum possible thermal energy. The Kelvin degree is the same size as a Celsius degree, so converting is simple: K = °C + 273.15.
Kelvin is used in physics and chemistry because thermodynamic laws are expressed in absolute temperatures. The ideal gas law PV = nRT requires temperature in Kelvin — if you doubled the temperature of a gas from 25°C to 50°C, the gas would not double in pressure because these Celsius values don't represent a true doubling. In Kelvin: 25°C = 298 K and 50°C = 323 K, an 8.4% increase, not 100%.
Notable temperature reference points
Some temperatures that are worth knowing across scales: The freezing point of water is 0°C / 32°F / 273.15 K. Human body temperature is 37°C / 98.6°F / 310.15 K. The boiling point of water is 100°C / 212°F / 373.15 K. Absolute zero is −273.15°C / −459.67°F / 0 K. The surface of the Sun is approximately 5,500°C / 9,932°F / 5,773 K.
A useful coincidence: Fahrenheit and Celsius read the same value at −40 degrees: −40°C = −40°F. Above this point, Celsius readings are always higher numerically than Fahrenheit; below it, Fahrenheit readings are higher. This crossover point is a handy sanity check when converting between scales.