Heating Effect of electric current:

The chemical reaction within the cell generates the potential difference between
its two terminals that sets the electrons in motion to flow the current through a resistor or a system of resistors connected to the battery.

Some of energy of the source (cell or battery) is used to maintain the current and consumed in usefulwork. Rest of the source energy may be expended in heat to raise the temperature of gadget. 

For example, an electric fan becomes warm if used continuously for longer time etc.

On the other hand, if the electric circuit is purely resistive, that is, a configuration of resistors only connected to a battery; the source energy continually gets dissipated entirely in the form of heat. This is known as the heating effect of electric current.