The degree Fahrenheit is a unit of temperature named for the German physicist Daniel Gabriel Fahrenheit, who proposed it in 1724. In the Fahrenheit scale of temperature, the freezing point of water is 32 degrees, and the boiling point is 212 degrees. Hence a degree Fahrenheit is 5/9ths of a Kelvin or degree Celsius.
Fahrenheit established zero degrees as the temperature at which an equal mixture of ice and salt melts (some say he took that fixed mixture of ice and salt that produced the lowest temperature); and ninety-six degrees as the temperature of a healthy human body. Initially, his scale had only contained 12 equal subdivisions, but then later he divided each division into 8 equal degrees ending up with 96. He then observed that plain water would freeze at 32 degrees and boil at 212 degrees.
His measurements were not entirely accurate, though; by his original scale, the actual freezing and boiling points would have been slightly different than 32 and 212. Some time after his death, the error was discovered, and it was decided to recalibrate the scale with 32 and 212 being the actual freezing and boiling points of plain water. This resulted in the healthy human body temperature being 98.6 degrees rather than 96.
The Fahrenheit scale was widely used in Europe until a switch to the Celsius (formerly centigrade) scale (for the conversion formulas, see that article). It is still used by the general population for everyday temperature measurement in the United States and a few other English-speaking countries.
Other temperature scales include the Réaumur (1730), Rømer[?] (1730+), kelvin (1862), and Rankine (ca. 1860). (Note that "kelvin" is lower-cased because it is an SI unit, even though it is named after a person).