Assuming you mean standard Energizer or Duracell Alkaline batteries. A chemical reaction between a metal Zinc usually and an "Alkaline" substance. More commonly known as a Base. (The opposite of an acid.)
The chemical reaction releases electrons on the negative terminal, and there is a lack of electrons on the positive terminal.
These excess electrons want to fill in the lack of electrons on the other side of the battery, but cant get there until you give it a circuit path through... lets say a flash light. Flowing electrons = electricity in all it's magic.