Latin name: Latrodectus mactans
The black widow is a poisonous spider found in most of the United States and in southern Canada. Unlike other spiders, the black widow is completely bald and a shiny jet black, with the exception of a red hourglass on its belly. It has long, spindly legs which support an oversized abdomen. It looks as wicked as it is. Black widows are known to eat their mates after mating. I do not know the evolutionary advantage of this.
If you are bitten by a black widow, you are not likely to die (only 1% of all cases), but are more likely to cramp up and go into cold sweats and nauseous convulsions as you are subjected to up to 48 hours of intense pain in the place you were bitten. Hopefully you will recover fully within 5 days. Black widow venom is 15 times as toxic as the venom of the prairie rattlesnake, but is given in a minute amount, which is why the mortality rate is so low.
If you are bitten, remain calm. If you can, collect the spider in a plastic bag for identification, and go to the hospital. You will be given antivenom or calcium gluconate to relieve the pain. If you are younger than 16 or older than 60, you may have to stay for observation as you ride it out. First Aid is of little help, but application of antiseptic to the bite is recommended to prevent infection.
My family had a black widow as a pet once. It wasn't intentional. My sister found it on some grapes we got at the grocery store. After the screaming stopped, my Mom managed to capture it, and put it in a terrarium. My family is like that. We named it Blackie. Blackie spun for us an irregular, silky web, on the twigs we stuck in there, which she used to trap the crickets we fed her.
We know that Blackie was a she, because she laid a couple of egg sacs while we had her in captivity. We were a little worried, because she was in a habitat made out of mosquito wire, which was small enough to block an adult spider, but not a horde of babies. Fortunately, the eggs never hatched. Blackie met an untimely end when her habitat was knocked onto the floor.
If you don't want to have a pet like Blackie, then you should keep the area around your house clear of debris and rubble, and keep the house itself clean and sealed of cracks. When you handle firewood, use gloves. Insecticides are also good against spiders, because the mist gets caught in the webs.
Thanks to http://www.ag.ohio-state.edu/~ohioline/hyg-fact/2000/2061A.html for the scientific and medical data.