From Portland Hikers Field Guide
The stinging nettle, Urtica dioica, is a perennial plant that grows at low elevations west of the Cascades. It prefers wet soil and is often seen next to creeks or stagnant bodies of water. Stinging nettles grow from 3 feet to occasionally as high as 8 feet. The leaves are vaguely heart shaped with serrated edges. Flowers are greenish, lack petals and hang from the stems looking almost like chains. There is vague, almost ironish smell to a patch of stinging nettles and they're sometimes smelled before they're seen.
The stems and leaves of stinging nettles are covered with small, sharp, fragile hairs. When the plant is brushed into, the hairs break, releasing formic acid. The acid causes an immediate burning sensation, followed by itching, which lasts for 1 hour to 24 hours. A chemical base, such as baking soda, will help neutralize the acid and reduce the itching. There are no long term effects and the the effects are not cumulative.
Stinging nettles have been eaten and used in tea for centuries. Cooking neutralizes the stinging effect.