The First Law of Thermodynamics applies the principle of conservation of energy to thermodynamic systems where the energy can be transferred both through work and through heat.  The first law helps highlight one of the critical misunderstandings students have with thermodynamics, that heat and temperature are the same thing.  This isn’t helped by the fact that we often describe the temperature of an object using terms like hot or cold, and if “hot” is a description of temperature, then many students mistakenly think that heat must be as well.  However, as the first law tells us, heat is a way that a system gains or loses energy.  Temperature, on the other hand, is a measure of the total energy of a system.  It is entirely possible for a system to lose energy as heat (meaning that `Q` is negative in the first law equation) but still increase its temperature, as long as the energy transferred into the system as work was greater than the energy lost as heat.