There are 6 officially recognised vitamins in total (A, B, C, D, E and K), but these have many sub-forms (e.g. B2, B6, B12 etc.), so they affect the body in many different ways, and like herbal extracts, their effects cannot be described in one word.
The most important of these properties is that vitamins have antioxidant effects. In a broad sense, antioxidants are enzymes, molecules or elements that play an important role in protecting against free radicals (substances that damage cells). Free radicals van be generated in our bodies by a wide variety of effects, including durinmg basic metabolic processes, stress, exercise, toxins, radioactive and UV radiation, and others. These free radicals can damage our DNA, among other things, which can lead to mutations (changes) over time, and mutations can lead to various diseases. All free radicals increase inflammation, so vitamins (antioxidants in general) also have anti-inflammatory effects.
In addition to these, vitamins act as catalysts for many metabolic processes, think of it like oil in a car, without it the engine won't run properly - we can't use energy without it.