Under normal conditions, the elementary particles of any substance (atoms of a chemical element, and molecules of a chemical compound) will move randomly to some extent. Because they move, they have kinetic energy.
Heat is the total kinetic energy that these particles have, or a measure of the transfer of such energy from one body to another. Temperature is the average of such energy and depends on the amount of substance present.
Temperature determines where heat will flow when two objects make contact. Heat will always flow from the object having the higher temperature to the object having the lower. Heat will not have a net flow between two objects whose temperature is the same.
Furthermore, if any two objects are each at thermal equilibrium with a third object, i.e. have the same temperature as that third object, then they must be in thermal equilibrium with one another. Ralph H. Fowler recognized this transitive property of thermal equilibrium as the Zeroth Law of Thermodynamics.
Absolute zero is the temperature at which all random motion of the elementary particles of a substance ceases. By definition, no substance can possibly be colder than this.
The absolute temperature of any substance is the difference between the measured temperature, on any given scale, and absolute zero.
Four different temperature scales are in common use today. Each one measures temperature with reference to one or more standard temperatures.
The Celsius or centigrade scale uses as its zero the freezing point of pure water. The boiling point on this scale is 100 degrees.
The Kelvin scale uses absolute zero as its zero. Its degrees describe the same interval as those on the Celsius scale, and therefore on this scale the freezing and boiling points of water are one hundred degrees apart.
The Rankine scale also uses absolute zero as its zero. Its degrees describe the same interval as those on the Fahrenheit scale.
To convert from Celsius to Fahrenheit, multiply the Celsius temperature by 9/5, and add 32, the Fahrenheit temperature of the freezing point of pure water.
To convert from Fahrenheit to Celsius, first subtract 32 from the Fahrenheit temperature and then multiply the result by 5/9.
To convert from Celsius to Kelvin, add 273.15 to the Celsius temperature. Absolute zero is 273.15 degrees Celsius below zero. (The Kelvin scale uses no degree symbol, and instead uses the letter K with no other symbol.)
To convert from Fahrenheit to Rankine, add 491.67 to the Fahrenheit temperature. Absolute zero is 459.67 degrees Fahrenheit below zero.