As described on whfoods.com:

Contrary to what their name implies, peanuts are not true nuts but a member of a family of legumes related to peas, lentils, chickpeas and other beans. Peanuts start growing as a ground flower that due to its heavy weight bends towards the ground and eventually burrows underground where the peanut actually matures. The veined brown shell or pod of the peanut contains two or three peanut kernels. Each oval-shaped kernel or seed is comprised of two off-white lobes that are covered by a brownish-red skin.