We all know that a hot body tends to increase the temperature of the bodies which surround it, while a cold body causes a decrease in temperature around it.  In stricter terms we could state that, when two substances at different temperatures are near to each other, there is an exchange of energy between them which tends to create thermal equilibrium, which is reached when both temperatures are equal.

According to the Law of  Conservation of Energy the net energy exchange between the two systems and the environment would be zero, and limiting ourselves to the simplest case, which is an ideal case, the situation could be expressed by saying that the heat lost by the hot system as it cools is exactly the same as the heat absorbed by the cold system as it heats up. 

In this ideal case we can make another simplification: that we only consider the hot and cold substances and not the containers, which would be considered ideal adiabatic containers, with walls which were perfect thermal insulators; the real life case which is most similar would be a thermos flask or a down sleeping bag.

In the visual we present the two substances exchange heat through the partition which separates them.