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The horsetails or scouring rushes (Equisetophyta, Sphenophyta, Arthrophyta, and Equisetaceae are among the names that have been used for this group) are now believed to form a monophyletic group with the ferns that is known as the "monilophytes" (although the position of the horsetails within the monilophytes is not yet fully resolved, they may be nested among other ferns); this clade, in turn, is the sister group to the seed plants (Pryer et al. 2001; Schneider et al. 2009 and references therein; Rai and Graham 2010 and references therein). There is just one extant genus, Equisetum, which includes around 15 extant species. Equisetum is nearly cosmopolitan (not native to Australia and New Zealand, but they are exotic weeds there). Many Equisetum have a high silica content and can be used to scour pots (explaining the name "scouring rush"). Horsetails have an extensive and diverse fossil record and several hundred million years ago widespread tree-sized relatives reached 30 m in height (even today, some Equisetum species can reach an impressive size--although nothing approaching 30 m!).
For more information on the biology of horsetails, see Chad Husby's website.