Stinging nettles (Urtica dioica) produce formic acid (similar to ant stings) which they hold in brittle hollow hairs. When you crush a plant, you break the hairs, causing the acid to burn your skin.
Simply brushing gently against a nettle is enough to break stinging hairs releasing the acid onto your skin which swells the surface and cause pain and itching. However painful, the sting is temporary, the pain subsides in 10 minutes or so.
In the Bottle Inn in Dorset England every year they have a stinging nettle eating competition. It looks horribly painful.
If you are stung, look around for large 'Doc' (Rumex obtusifolia) leaves which ease the pain of nettle stings when rubbed against the skin
Despite their fearsome reputation the nettle has many uses. Nettles are highly nutricious and grow so abundantly that ignoring them as a source of food is just wasteful.
When young the leaves of this abundant weed can be cooked and eaten like spinach or used to make nettle tea which is ascribed with many health giving properties.
In his excellent book Wild Food, John Phillips notes that a chemical change occurs in nettles around midsummer which makes them particularly bitter. Therefore nettles are best eaten in the spring only.
Crushing the leaves disables the stings so nettles can even be served in salads... if you are careful.
It is also true that Stinging Nettle stems have long fibers of good strength and can be used as a linen substitute.
Nettles can be made into wine. Here is a nettle wine recipe I have tried and successfully tested for you.
Above all, in the garden nettles make fantastic fertilizer. A lidded bucketful
of nettles covered in water and left to stew for a week or two produces an
all purpose liquid feed of high quality. Watered down 10 to 1 it can benefit
the entire garden and particularly tomatoes.