The plants are characterized by slender, green, often drooping branches that are deeply grooved and that bear, at intervals, whorls of tiny, scalelike leaves. Long plant hairs that protrude from the grooves are thought to function as protective structures for the stomates (microscopic pores to the internal tissues), which are located along the side walls of the grooves. These structural features serve as adaptations to the dry conditions of the coastal strands and poor soils where these plants often grow. The structurally reduced flowers are separately male and female; both sexes may occur on the same plant (monoecious condition) or on separate plants (dioecious condition). Male flowers occur in elongated, slender, erect clusters (catkins or spikes), usually at branchlet tips, and each consists of a single pollen-producing stamen, together with two small, scalelike floral leaves (sepals or bracts) and two smaller scalelike structures called bracteoles. The female flowers occur in dense globular clusters that become woody and conelike at maturity, the woody segments enclosing the seeds. Each female flower is a petalless, two-chambered structure (pistil) with two ovules, both in the same chamber. Two long style branches or stigmas extend from the upper end of the pistil beyond the flower cluster. Pollination is by wind.

Older classification systems held this family to be the most primitive of dicotyledonous plants, but the flowers and other primitive-appearing characteristics are now considered to be reduced rather than primitive. Several species of Casuarina, especially C. equisetifolia, are valued for their hard, dense, yellowish to reddish brown wood, which is strong and reputed to be resistant to termite attack. Beefwood and ironwood are common names that reflect the colour and hardness of this wood.