Achillea millefolium: the genus is named for the Greek hero Achilles in the Trojan War of the Iliad who was said to have saved the lives of soldiers by applying yarrow to their wounds. His plant knowledge apparently didn't save him when he got shot in the heel with an arrow, but at least he got a tendon named after him. That's pretty cool.
Yarrow has a huge range - it's found throughout North America, Asia, and Europe. You can find it growing from sea level all the way up to alpine areas; it's everywhere, and it's useful. This plant is considered a styptic, which means that it's capable of stopping bleeding when applied to a wound, a quality it gets from the alkaloid Achilleine. Cultures from across the globe have used it to treat wounds, reduce fevers, deal with diarrhea, repel insects, and more... if you want a great detailed article, here's one.
The plant can be recognized from it's feathery fern-like leaves and flat-topped dome of white flowers. Millefolium translates as thousand-leaved, and the extensive surface area of these intricate leaves can accumulate vehicle particulates, so don't harvest it by roadsides.
When used in teas and tinctures, both the flowers and leaves are harvested when the plant is flowering. As with all things in life, moderation is key: if consumed in excess yarrow can have a phototoxic effect, making a person sensitive to sunlight. It should also not be consumed by pregnant women because it can stimulate uterine contractions.
Most importantly, people in medieval Europe used yarrow to make beer prior to the popularity of hops. Yarrow was blended with wild rosemary and bog myrtle to make what is called a Gruit beer, which is said to have more stimulating effects than the sedative hops.
