Thermodynamics: the study of systems involving energy in the form of heat and work.
The First Law of Thermodynamics: Q = DU + W
Q represents heat added to a system (or removed if it is negative)
U is the internal energy of the system (the energy associated with the motion of the atoms and/or molecules), so DU is the change in the internal energy. This is directly proportional to the change in temperature.
W is the work done by the system.
The First Law is often written as DU = Q - W