Cell – the basic building block of a battery.
Batteries can contain just a single cell or multiple cells. Batteries are often referred to as cells and a single cell as a battery (even though technically this is incorrect).
A cell is made up of a cathode and an anode immersed in an electrolyte. What these three elements are made from defines the voltage of the cell. A cell in a car battery, for example, which is made up of lead alloy cathodes and anodes immersed in sulfuric acid produces a voltage of just over 2 when fully charged.
2 volts would not provide the power needed to start an engine, so six cells are connected in series to create a 12 volt battery.
To understand how a single cell works see How a battery works.