The cherry gall (Cynips quercusfolii) is the product of the egg laying of a small species of gall wasp. It lays its eggs on the underside of oak (quercus) foliage (folii) and they develop there, sometimes one, sometimes a small collection. They start pale green (like the one I photographed) and become red as they mature and that is when they begin to look like cherries.
As with similar galls a single 'fruit' has several chambers in it, each with a single larva. When the leaf falls, so too does the gall and the larvae eat their way out and live over winter in the leaf litter before laying eggs in the spring.
It is interesting that galls on our common English or pedunculate oak are usually smooth but those laid on the introduced sessile oak are rough or warty; I guess that must be something different in the chemistry of the two species of oak?