Achillea millefolium or yarrow is a flowering plant in the family Asteraceae, native to the Northern Hemisphere. In Spanish-speaking New Mexico and southern Colorado, it is called plumajillo, or "little feather", for the shape of the leaves. In antiquity, yarrow was known as herbal militaris, for its use in staunching the flow of blood from wounds. Other common names for this species include common yarrow, gordaldo, nosebleed plant, old man's pepper, devil's nettle, sanguinary, milfoil, soldier's woundwort, thousand-leaf (as its binomial name affirms), and thousand-seal.
Yarrow has seen historical use as a medicine, often because of its astringent effects. Decoctions have been used to treat inflammations, such as hemorrhoids, and headaches. Confusingly, it has been said to both stop bleeding and promote it. Infusions of yarrow, taken either internally or externally, are said to speed recovery from severe bruising. The most medicinally active part of the plant is the flowering tops. They also have a mild stimulant effect, and have been used as a snuff. Today, yarrow is valued mainly for its action in colds and influenza, and also for its effect on the circulatory, digestive, excretory, and urinary systems. In the nineteenth century, yarrow was said to have a greater number of indications than any other herb.
Now a small personal note, I love the leaf structure of this plant even more than the flowering ability...but best of all it survives in a "dog heavy" yard and each spring it fights it way back to survive.
(Information taken in part from wikipedia....thank you very much)