A battery is a way to convert chemical energy into electrical energy. Inside the battery are the ingredients for a chemical reaction, but arranged so that the reaction can only happen when a current flows. In the battery that we just made, zinc metal is turning into zinc ions at the surface of the zinc washer where it touches the wetted paper. The zinc atoms in the metal have no charge, while the zinc ions have positive charge. Thus this chemical reaction requires the addition of charge; it is delivered by the current carried by the wire. At the surface of the brass washer, hydrogen ions (supplied by the vinegar) are giving up their charge to become hydrogen gas. The released charges are carried away by the current. So there is a current out of the brass and into the zinc for the two reactions to take place. Inside the battery, the current takes the form of zinc ions moving away from the surface where they were created, and hydrogen ions moving towards the brass surface where they turn into hydrogen gas. An alternate description, involving the electrons

The directionality of a battery -- which end is positive, and which negative -- is determined by the chemical reactions involved: when a battery is in a circuit, current flows from the positive end to the negative end through the circuit, and the chemical reactions within the battery are releasing energy. Some batteries are rechargeable, which means that forcing a current in the positive end of the battery will reverse the chemical reactions and cause electrical energy to be converted into chemical energy.

Chemical reactions that are facilitated by the presence of an electrical current occur in other places besides batteries. Perhaps you have touched a filling in your teeth with a metal object, or noted the funny effect if you bite aluminum foil -- these are the consequences of making an accidental battery in your mouth! Coating iron with zinc protects it from rusting because it creates a battery that drives away the ions that might attack the iron, and which reverses the rusting reaction as fast as it happens (this does erode away the zinc, eventually). This trick of having a "sacrificial" metal piece to prevent corrosion of important parts is also used in water heaters and other applications.