Yarrow is Achillea millefolium, and both halves of the name are informative. Millefolium, thousand-leaf, describes the finely divided feathery foliage exactly. Achillea records the tradition, reported by Pliny among others, that Achilles used the plant to treat his soldiers' wounds, a story that fixes the plant's reputation in the classical period, whatever its historical value.
The wound reputation is not empty. Yarrow has a documented styptic action, and the English folk names (soldier's woundwort, staunchweed, nosebleed, carpenter's herb), all record the same use across centuries. It contains achilleine, along with tannins and flavonoids, and a plausible mechanism involving shortened clotting time has been described. This is at the more credible end of traditional wound care.
The plant is also an aromatic bitter and a traditional diaphoretic, taken hot to break a fever by sweating, which is what Culpeper describes.
Two cautions belong in a factual account. Yarrow contains small amounts of thujone. And it is a notable cause of contact dermatitis in sensitive people, and of photosensitisation, sufficient that it is a recognised occupational irritant among those who handle it in quantity. It is also easily confused in the field with several dangerous umbellifers at the rosette stage by inexperienced foragers, though the foliage and flower structure are quite distinct once known.
Yarrow is not an umbellifer, despite appearances. The flat-topped head is a corymb of many small composite flowers, and the plant is in the daisy family, each apparent petal is a complete ray floret. The resemblance to hemlock and cow parsley is convergent and superficial.