The Plant Book mentions that there are about 600 species of oaks in the world, mostly in the temperate regions of Eurasia and North America, but some tropical species also in Mexico, Southeast Asia, and New Guinea. North America has about 60 species classified. Most are medium to large deciduous trees, but there are some evergreen species and species that are shrubs down to only 3 feet high. The plants are monoecious with tiny, yellowish, staminate (male) flowers in hanging, clustered aments near the ends of twigs and the tiny, yellowish, pistillate (female) flowers as solitary or in small spikes in new leaf axils. The fruit is an acorn. Scaled buds are clustered close together at the ends of twigs and the rest are alternate along the twigs.

There are two groups of oaks: the White Oak Group and the Black Oak Group. The first group has leaf lobes or marginal teeth that are rounded, not tipped with bristles. Their acorns mature in one year and the acorn shells are hairless inside. The second group has sharp pointed, bristle-tipped lobes or marginal teeth or leaves with smooth margins often with a bristle-tipped or spiny apex. Their acorns usually mature in two seasons and have hair inside the shells.

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