The laws of thermodynamics explain how heat energy behaves in the 
universe.

Since there are four laws, one might imagine they're known
individually as the first, second, third, and fourth Laws
of Thermodynamics.  But actually, the fourth law was described
after the first three; and it was deemed to be so fundamental
that it was dubbed the "Zeroth Law," so that it would be 
considered foremost in importance.

Here are the laws, expressed in common parlance.

The Zeroth Law:  A system with heat energy will always try
to reach thermal equilibrium.  One consequence of this law is that
heat always flows from greater concentration
to lesser concentration, unless forced to do otherwise.

The First Law: The total amount of energy in the universe, or an
isolated part of the universe, always stays the same.  This is known
as the "Conservation of Energy."

The Second Law: The total amount of disorder in the universe, or in
an isolated part of the universe, always increases.

The Third Law: There exists a temperature, which we call "absolute
zero," where heat energy does not exist.  However, there is no way
in the universe to achieve that temperature in any system.