Food additives can be divided into several groups, although there is some overlap between them.
Acids are added to make flavors “sharper”, and also act as preservatives and antioxidants. Common food acids include vinegar, citric acid, tartaric acid, malic acid, fumaric acid, and lactic acid.
Acidity regulators are used to change or otherwise control the acidity and alkalinity of foods. Anticaking agents
Anticaking agents keep powders such as milk powder from caking or sticking.
Antifoaming agents reduce or prevent foaming in foods. Antioxidants Antioxidants such as vitamin C act as preservatives by inhibiting the effects of oxygen on food, and can be beneficial to health.
Bulking agents such as starch are additives that increase the bulk of a food without affecting its taste.
Food colourings are added to food to replace colours lost during preparation, or to make food look more attractive. Colour retention agents In contrast to colourings, colour retention agents are used to preserve a food’s existing colour.
Emulsifiers allow water and oils to remain mixed together in an emulsion, as in mayonnaise, ice cream, and homogenized milk.
Flavours are additives that give food a particular taste or smell, and may be derived from natural ingredients or created artificially.
Flavour enhancers enhance a food’s existing flavours. They may be extracted from natural sources (through distillation, solvent extraction, maceration, among other methods) or created artificially.
Flour treatment agents added to flour to improve its colour or its use in baking.
Glazing agents provide a shiny appearance or protective coating to foods.
Humectants prevent foods from drying out.
Tracer gas allows for package integrity testing to prevent foods from being exposed to atmosphere, thus guaranteeing shelf life.
Preservatives prevent or inhibit spoilage of food due to fungi, bacteria and other microorganisms.
Stabilizers, thickeners and gelling agents, like agar or pectin (used in jam for example) give foods a firmer texture. While they are not true emulsifiers, they help to stabilize emulsions.
Sweeteners are added to foods for flavouring. Sweeteners other than sugar are added to keep the food energy (calories) low, or because they have beneficial effects for diabetes mellitus and tooth decay and diarrhea.
Thickeners are substances which, when added to the mixture, increase its viscosity without substantially modifying its other properties.
For a complete list: http://en.wikipedia.org/wiki/List_of_food_additives