The term "chemical reaction" refers to any process that changes a set of chemical substances to another. That is, the substances that are put into a chemical reaction are different, on a molecular level, from those that result.
Essentially, in a chemical reaction, the atoms that make up the involved chemicals rearrange to form new ones that are more stable under current conditions. The only way to definitively tell that a chemical reaction has occurred is by doing chemical analysis in a lab. However, a good heuristic that is commonly used to identify chemical reactions is that chemical reactions typically produce heat, light, color changes, gas, a precipitate or some combination thereof. For example, the reaction between the acetic acid in vinegar and sodium bicarbonate produces carbon dioxide gas, which was not present before the reaction.