The two are related, but are not the same thing. Key limes are slightly smaller than regular limes — about the size of a golf ball — and they have thinner skin. They are more fragile, so they are less likely to be seen in the supermarket, but they are prized for their aromatic flavor. Although the flavor of the two is different, with key limes having a more tangy flavor, they can be used interchangeably in most recipes.