In simple terms the battery can be defined as a closed container filled with chemicals, which through electrochemical reactions convert chemical energy into electricity.
A battery of any type has two poles of the positive (+) and negative (-)
If a wire is connected between the two poles, the electrons will move through the wire from the negative pole to the positive pole. The chemical energy that is stored in the battery is moving the electrons (that is electricity). Therefore the chemical energy is transformed into kinetic and then into heat (especially in the wire connecting the poles) and the battery "empties". Heat is generated when the moving electrons collide with ions of the wire, and increase the kinetic state of the ions.
Usually between the two poles of the battery something useful is connected e.g. a light, a tape recorder, a radio, etc. This device uses the electricity produced from the battery.
The chemical reactions inside the battery, start as soon as the two poles are connected by a wire. That is closing the circuit.
The first electric battery created by Alessandro Volta in 1800.