The horsetails are vascular plants, comprising 15 species of plants in the genus Equisetum. This genus is the only one in the family Equisetaceae, which in turn is the only family in the order Equisetales and the class Equisetopsida. This class is often placed as the sole member of the Division Equisetophyta (also called Arthrophyta in older works), though some recent molecular analyses place the genus within Pteridophyta, related to Marattiales. The molecular data, however, are somewhat ambiguous as of yet. Other classes and orders of Equisetophyta are known from the fossil record, where they were important members of the world flora during the Carboniferous period.

The name horsetail arose because it was thought that the stalk resembled a horse's tail; the name Equisetum is from the Latin equus, "horse", and seta, "bristle". Other names, rarely used, include candock (applied to branching species only), and scouring-rush (applied to the unbranched or sparsely branched species). The name scouring-rush refers to its rush-like appearance and because the stems are coated with abrasive silica that led them to be used for scouring cooking pots in the past.

The genus is near-cosmopolitan, being absent only from Australasia and Antarctica. They are perennial plants, either herbaceous, dying back in winter (most temperate species) or evergreen (some tropical species, and the temperate Equisetum hyemale). They mostly grow 0.2-1.5 m tall, though E. telmateia can exceptionally reach 2.5 m, and the tropical American species E. giganteum 5 m, and E. myriochaetum 8 m.