A rock is a naturally occurring aggregate of minerals and/or mineraloids. Rocks can be picked up and thrown using the Throwingskill or employed as bludgeons using the Unarmed skill. Besides rocks, there are also gold nuggets and uranium ore which can be used in the same way.
The locations rocks can potentially be found in both Fallout games are too numerous to list; suffice to say that if someone ever finds themselves looking for rocks, they will not have to go far. Specifically, children use them as a weapon.