A horsetail is a member of the group Sphenopsida, a subdivision of the plant kingdom which reached its evolutionary peak during the Carboniferous period. Present-day sphenopsids comprise a single genus, Equisetum, which are green rush-like weeds found throughout the world except Australia and New Zealand.

Horsetails are spore-bearing perennials related to ferns and clubmosses. They have creeping rhizomes; an annual, distinctively jointed stem; a whorl of scale-like leaves around each joint; and a cone-like strobilus, bearing the spores, at the tip.