Horror of horrors, this mushroom is responsible for most of the cases of mushroom poisoning worldwide. The most common victims of this mushroom are amateurs who don't care to read a field guide or east asian immigrants who mistake it for the edible Volvariella volvacea, or paddy straw mushroom. At any rate, it's vitally important that you learn to recognize this mushroom if you ever plan to head out into the woods looking for something to munch on (for whatever reasons). The genus Amanita should be avoided in general as species aren't always easily recognized beyond the microscopic level and there are other poisonous species in the genus such as the destroying angel (although that warning really won't deter Amanita muscaria hunters).
Looking out for identifying features of these guys are pretty easy. It has a sac-like volva (a bit like a cup that a mushroom grows out of), a white annulus (ring) and spore print, and a smooth cap margin. The cap is either greenish, light brown, yellow or off white, but color is a bad indicator due to the huge variance found. The cap is about 4-14 cm long and the stem can be 5-18 cm long and 1-3 cm thick. The gills are free, close together and white to cream colored. It's said to have a nice taste from those who have survived poisoning, although you probably won't want to try that yourself. It can be frequently found under live oak trees and in fact has a symbiotic relationship with many different trees by providing them with nutrients (see mycorrhizal fungi). Although it's believed to have originated in Europe this mushroom can now be found in North America, Australia, South America, and Africa as it was easily transported along with the trees it was living with. It's frequently found blooming in deciduous woods during the fall season, but some reports of springtime fruiting are reported.
The toxic nature of of this species is caused by two classes of toxins: phallotoxins and amanitins. Most notable is Alpha-amantin, a small protein that attacks RNA polymerase in the liver. Victims may get sick up to 24 hours after digestion with symptoms like vomiting, diarrhea and cramps. These symptoms will remiss, which may mislead the victims into thinking they're fine. However, about a week later liver and kidney failure will ensue. Treatment usually involves administering huge amounts of penicillin (as a liver stimulant), cephalosporin, and possibly a liver transplant. It is said that victims rarely return to their previous levels of health.