I've just pulled this off of
http://encarta.msn.com/encyclopedia_761567558/Nettle.html
It's quite a good summary of the quantity and types of nettles
Nettle, common name for a flowering plant family, and for its representative genus. The family comprises over 1000 species placed in 45 genera. Their stems and leaves are often armed with hollow hairs filled with a stinging liquid. The brittle, gland-tipped hairs are easily broken, and the liquid, if injected into the skin, produces a burning sensation that may be of temporary or long duration. The sting of the stinging nettle lasts only a few minutes, whereas that of an Australian species produces such a severe reaction that death has been reported in a few cases. Nettles are of little economic importance. Ramie fiber, or China grass, obtained from a nettle species of Southeast Asia, is finer than hemp and stronger than cotton, but it is difficult to extract.
The family belongs to an order, mainly tropical in distribution although with many temperate members as well, that has traditionally included about 2400 species placed in six families, although recent information places 6000 species divided into three families in the order. Other important families in the order are the mulberry family, with about 3000 species, and the elm family, with about 2000 species. The elm and mulberry families are mostly woody, whereas the nettle family is mostly herbaceous. The flowers, usually unisexual (either male or female), are small and lack petals. They are often wind-pollinated, and each female flower produces a single seed.
Scientific classification: Nettles make up the family Urticaceae of the order Urticales. Its representative genus is Urtica. The stinging nettle is classified as Urtica dioica. Ramie fiber, or China grass, is obtained from the species classified as Boehmeria nivea.
