Heat is the energy that is operated because of differences in temperature among the body and the surrounding. The unit of heat energy is Joule.
The basic difference among study of mechanics and thermodynamics is in part of energy move into and out of the body. In mechanics the energy move into and out of the system happens because of work done by force. In thermodynamics the energy move may take place, in addition to the work given by force, by heat transfer because of temperature differences among the body and environment. We have a sign convention for these two parts of energy transfer.