Oak, (genus Quercus), any of about 450 species of ornamental and timber trees and shrubs constituting the genus Quercus in the beech family (Fagaceae), distributed throughout the north temperate zone and at high altitudes in the tropics.

Oaks can be separated into three groups, sometimes considered subgenera: white oaks (Leucobalanus) and red or black oaks (Erythrobalanus) have the scales of the acorn cups spirally arranged; in the third group (Cyclobalanus) the scales are fused into concentric rings. White oaks have smooth, non-bristle-tipped leaves, occasionally with glandular margins. Their acorns mature in one season, have sweet-tasting seeds, and germinate within a few days after their fall. Red or black oaks have bristle-tipped leaves, hairy-lined acorn shells, and bitter fruits, which mature at the end of the second growing season. - britannica