dU = mCmdT         (change in energy = mass * heat capacity per unit mass
                           * change in temperature)

  dU = VCVdT         (change in energy = volume * heat capacity per unit volume)

All matter stores energy in the form of heat.
When a substance is not near its phase boundaries (the lines on its phase
diagram that separate solid, liquid and gas), the amount of energy stored is
proportional to the temperature of the substance.  It is also proportional to
the heat capacity of the substance, as described by the equations above.
(Sometimes chemists will refer to the change in heat energy as dQ rather than
dU, but the ideas are the same.) 

The first equation describes the heat capacity of a substance as it relates 
to mass, and is usually used for solid or liquid phases.  The MKS unit of
this heat capacity is "Joules per kg per degree K."  The second equation
describes the heat capacity of a substance as it relates to volume, and
is usually used for the gaseous phase.  In that case, the MKS unit is  
Sometimes, "Joules per cubic meter per degree K."

Heat energy is often expressed in a unit called a calorie.  One calorie is
equal to 4.2 Joules.  The heat energy stored in the food you eat is labeled
as "Calories," which is actually 1,000 "calories." Scientists usually refer
to the food "Calorie" as a "kilocalorie," or Kcal for short.