Flavor (or flavour in British English) is the main quality of anything which affects the taste. When someone remarks on the flavor of something they are trying to describe the specific way something tastes.
Flavor can also be used to describe the act of adding a taste alteration (to change the taste) to something. This is usually done by adding spices or sugars, though in processed food there are oftentimes artificial (fake) flavors. A person who develops natural and/or artificial food additives is called a flavourist or flavorist (US English).