An odor is anything you can smell — like a scent or an aroma. If you sniff a sour odor in your gallon of milk, you should probably pour it down the sink instead of drinking it.
Most people are more likely to use the word odor for unpleasant smells, like the odor of your uncle's stinky cigars or the musty odor in your grandmother's attic. You can also use the word odor in a figurative way, to mean a feeling or impression given by something: "The whole bank seemed full of the odor of money and greed." Odor is Latin, meaning "a smell or a scent."