Where does odor come from?

It depends what sort of odor you are asking about. Generally, smell is caused by tiny fragments of a substance being identified by receptors in your nasal passage (nose). If you are talking about the cause of this odor, this can come from a variety of different situations. Body odor, contrary to common belief, is not actually the smell of your sweat but of the bacteria who ingest nutrients found in your sweat. This is why body odor is only particularly fragrant after extended periods of non-washing.