dU = TdS + PdV    (change in energy = [temperature * change in entropy]
                           + [pressure * change in volume])

This equation describes the combined
effects of the first and second Laws of Thermodynamics.  The first term of
the equation deals primarily with the energy associated with phase changes, 
where the entropy of a substance in a given volume changes.  (True ideal 
gases do not change phase, so entropy is usually ignored.)  The second term 
deals primarily with the energy associated with volume changes in gas, where 
the temperature and entropy per particle remain constant.  

When a phase change occurs - that is, a substance moves from one region of its
phase diagram to another - the entropy of that substance changes even as its
temperature stays the same.  A substance in solid phase has low entropy; in
liquid phase, it has medium entropy; in gas phase, it has high entropy.  So
looking at the equation dU = TdS + PdV, the total amount of heat energy in a
substance must increase if it changes phase from solid to liquid, solid to gas,
or liquid to gas.  Conversely, the heat energy must decrease if it changes phase
from liquid to solid, gas to liquid, or gas to solid.  

The amount of heat energy that must be exchanged to move from solid to liquid
phase, or from liquid to solid phase, is called the heat of fusion.  The amount
of heat energy that must be exchanged to move from solid or liquid to gas, or
from gas to liquid or solid, is called the heat of vaporization. This energy
is usually given in units of energy per mass; in MKS units, that's Joules per
kilogram.