A member of the Urticaceae family (Urtica is from the Latin urere - 'to burn') with about 500 species worldwide, some in the Far East can produce a sting which burns for days. It is a perennial reaching about 1.2 metres in height.
The dark green leaves oppose each other on the stem, they are 5 to 15 cm long and 2 to 5 cm wide, serrated and with a tapered end. Tiny stiff hairs on the leaves are hollow enabling them to inject a cocktail including formic acid and histamine which causes a painful rash. An old remedy for the sting is to use Dock leaves or perhaps the more soothing, clear, slimey sap found at the base of the leaf stalk - rosemary, mint and sage leaves can also be used to rub over the affected area. This stinging does not occur when they are dried or cooked, although there may be some skin irritation from the dried hairs.
The flowers appear in the summer, they are tiny, greenish or greenish-white hanging down in clusters just above where the leaves attach to the stem (leaf axils) and are pollinated by the wind.

Continual close mowing will kill them, but the thick yellow roots remain for a year or two and will regenerate if mowing stops - eventually they die and grass will establish. I find that if they are pulled at the mature stage the thick surface roots tend to come away too and the nettles may not return or do so in reduced numbers. Although the finer roots go down fairly deeply, if the network of thick, yellow underground stems just under the surface are removed, any regrowth is sparce and weak.
A systemic or translocated herbicide will kill them, eg Gylphosate.

Nettles have been used for millennia in herbal medicine and as a pot herb. Archaeological evidence shows that they have been eaten as long ago as 6,000 years - a meal of nettles, crushed wheat with added hedgehog for flavour cooked like a pudding. Young shoots contain about the same amounts of beta-carotene and vitamin C as spinach and other greens. Only shoots from young plants before they bloom should be used, as after this point toxins develop which can damage the kidneys. A tea can be brewed from the tips of the young fresh leaves infused in water which is just off the boil (this preserves more of the beneficial properties). They can also be dried for later use, either naturally or in a warm oven (as it cools after use).
Extracts from the the root are used to treat inflammation of the prostate and have been shown to inhibit the growth of prostate tissue. To keep a supply of young shoots for culinary or medicinal use, mow the patch of nettles before they start to bloom and wait for the regrowth, allowing time for the roots to be replenished to prevent them from dying out.

The leaves contain lots of nitrogen and can be added to the compost heap to promote decomposition or used to make a liquid manure. This juice can be diluted to make in insecticidal spray (if a bit smelly) and a bunch of nettles hung in the kitchen deters flies. The stalks contain strong fibres which have been used to make cloth, sails or twine.
The stinging properties of the leaves have been utilised in the past. Medieval monks flagellated themselves as a penance, others have done this to ease rheumatism and arthritis pain, like a heat rub - the counter-irritant effect. Roman soldiers used this warming effect to adapt to the cold, damp, British climate, rubbing the Nettles on their arms and legs.
They are also the perferred food source for many of our native butterflies such as the Small Tortoiseshell and the Peacock. So if possible leave a patch in a corner somewhere.
See also Hedge Woundwort and Common Hemp-nettle which have similar leaves, but no sting.

Nicholas Culpepper
(17th century astrologer-physician)
" The roots or leaves, or the juice of them, boiled and made into an electuary with honey and sugar, is a safe and sure medicine to open the passages of the lungs, which is the cause of wheezing and shortness of the breath. It helps to expectorate phlegm and to raise the imposthumed pleurisy. As a gargle it helps the swelling of the mouth and throat.
A decoction of the leaves provokes the courses and urine and expels gravel and stone. It kills worms in children, eases pain in the sides and dissolves wind in the spleen.
The seed taken as a drink is remedy against the bites of dogs and the poisinous qualities of Hemlock, Henbane, Nightshade and Mandrake. The bruised seed or leaves put into the nostrils takes away the polypus. The juice of the leaves or a decoction of the root is used as a wash for fistulas and gangrenes and for corroding scabs or itch."