Temperature and heat

Thermal equilibrium:

By touch we can distinguish between a hot body and a cold body. If a hot system is brought into contact with a cold system , both systems will under go changes. The degrees of hotness and coldness of the respective systems will change , and finally a stage will be reached when the hotness (or coldness) of the two bodies does not change anymore. Then the two bodies can be said to be in thermal equilibrium.

The Zeroth law of Thermodynamics:

The Zeroth law state that if two systems taken separately can each be in equilibrium with a third system , the two can be at equilibrium with each other. Thus any two systems which are in thermal equilibrium have a property in common : this property can be defined as temperature. 

Heat: 

When two systems having different temperatures are in contact , the change toward equilibrium are brought about by an interaction across the boundaries which are in contact. The interaction which has taken place in this instance is called a heat interaction. Heat and work are both boundary interactions , and any interaction which cannot be classed as a heat interaction is a work interaction. 

Measurement of heat:

Heat was once measured using a set of units different from that for work. In the British Systems of units it was measured in terms of the Btu( British thermal unit ) which is the heat necessary to increase the temperature of 1 pound of water by 1°F and in the CGS system the unit is the calorie which is the heat necessary to increase the temperature of 1 gram of water by 1°C . In the SI , both heat and work are measured in joules(J).