Temperature
The hotter something is the faster the atoms are jiggling around. Let's say we have two objects, one hot, the other cold, and we bring them together. There's more jiggling around going on in the hot object.
There is an exchange of kinetic energy between the two objects.
Given enough time this energy will become evenly distributed between the two objects. The atoms in the hotter object will have slowed down while the atoms in the colder object will have speeded up. Thus we see that heat is always passed from the hotter to the colder object.
One of the earliest ways of measuring temperature was the thermometer that is based on the expansion in the volume of certain liquids with an increase in temperature. Gabriel Daniel Fahrenheit invented the mercury thermometer in 1714. Before that alcohol was used. Standardizing of a temperature scale and calibrating thermometers allows one to compare the temperatures of systems that exist at different times and places.
Temperature scales
The Fahrenheit scale is in general usage in English-speaking countries. Chemists usually use the Celcius scale, established by Anders Celcius (1701-1744), which assigns zero degrees to the freezing point of air-saturated water and 100 degrees to boiling water at one atmosphere. In the Fahrenheit scale water freezes at 32 degrees and boils at 212 degrees.
So a temperature difference of 100° C is 180° F. The ratio 100/180 can be reduced to 5/9. To convert from degrees F to degrees C we have to subtract 32 and multiply by 5/9. To convert from degrees C to degrees F we have to multiply by 9/5 (which gives you Fahrenheit sized degrees) and then add 32 (which adjusts for the offset).
Let's convert the boiling temperature of water as 212° F to Celcius.
212 - 32 = 180. 180 x 5/9 = 100.
A comfortable room temperature is 72° F. Let's convert that to Celcius.
72 - 32 = 40. 40 x 5/9 = 22.2.
25° C is used as a "standard" temperature. Let's convert that to Fahrenheit.
25 x 9/5 = 45. 45 + 32 = 77.
The 32 that we add or subtract is in degrees Fahrenheit, so we always add or subtract to the temperature in Fahrenheit sized degrees.
Absolute temperature
It was observed that as a gas was cooled that the volume decreased in a regular manner such that, if the process were continued, the volume would go to zero at about -273° C. Thus it was conceived that this would be the lowest possible temperature since all movement would cease at this temperature. This temperature was called absolute zero. A more accurate value is -273.15° C. Lord Kelvin devised a temperature scale based on the Celcius degree that began from absolute zero. The scale is named after him. In the Kelvin scale water freezes at 273.15 Kelvin or 273.15K. The scale has the property that 100K is twice as hot as 50K. This is not true of the Celcius or Fahrenheit scale. The so-called standard temperature of 25° C is 298.15K. The "absolute" property of the Kelvin scale makes the writing of thermodynamic functions simpler.