The wool sower gall is an excellent example with a complex life cycle. The gall to your right is the size of your finger tip and looks like a cotton ball with distinctive pink spots. Like many tree galls, this is produced by a Cynipid (sin-a-pid) gall wasp. These wasps are from 1-8 mm in length and many have a distinct hump on the back. They lay their eggs on a specific plant and the eggs produce the grubs whose secretions cause the gall formation. The gall in turn provides both protection and nutrition. How it does it remains a mystery. The most common host plants for the Cynipids are oak trees. see Wikipedia on Gall Wasps
Look for them on white oak trees in the spring. They don't cause any significant harm to the tree. You can put a fresh wool sower gall in a plastic bag and wait for one to three weeks for the tiny wasps to emerge. Keep it out of the direct sun so it doesn't cook, and remember that in spite of the name wasp, this family is totally harmless.
From North Carolina University