A battery is essentially a can full of chemicals that produce electrons. Chemical reactions that produce electrons are called electrochemical reactions. In any battery, there are two terminals positive (+) and negative (-). Electrons are collected on the negative terminal of the battery.
If one connects a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal. The electrochemical reaction moves electrons from one pole to the other. The actual metals and electrolytes that are used control the voltage of the battery. Different reaction has a characteristic voltage.