Spiders make up the scientific order Araneae which is a division of the class Arachnida. It is not accurate to label spiders as "poisonous" because that label is only for substances that are ingested. Spiders are instead more correctly labeled as venomous. There are 109 different spider families that are subdivided into over 40,000 different species. However, there are only really 10 known genera that cause significant harm to humans.

...
A close-up of a Black Widow Spider.

False Widow Spiders

...
A False Widow Spider.

There are members of the genus Steatoda that are called False Widow Spiders. They are often mistaken for Widow Spiders. The members who cause arachnidism (a medically significant bite by a venomous spider bite with varying effects) are the S. grossa in Australia (commonly known as the brown house spider, the cupboard spider and the dark comb-footed spider) and S. nobilis in England (commonly known as the biting spider).

Funnel Web Spiders

...
A Funnel Web Spider.

The only members of the genus Atrax are called Funnel Web Spiders because the toxins that they release are known as atracotoxins (ACTX). Their cousins in the genus Hadronyche are called known as Funnel Web Spiders. Since it is only located in Australia, it is sometimes known as the Australian funnel web spider.

Hobo Spiders

...
A Hobo Spider.

The North American members of the genus Tegenaria are known as Hobo Spiders. They are related to both the Funnel Web Spiders and the Recluse Spiders.

Mouse Spiders

Spiders of the genus Missulena are known as Mouse Spiders. They are found throughout their indigenous Australia. However, there is one family species (M. tussulena) that can be located in parts of South America, particularly in Chile.

Six-Eyed Sand Spiders

...
A Six-eyed Sand Spider.

Six-eyed Sand Spiders, of the genus Sicarius, live mainly in the African deserts. They are cousins to the Recluse Spiders. There arachnidism is so severe that its effects are equated to those of an untreated rattlesnake bite. There is no known anti-venom.

Recluse Spiders

...
A Brown Recluse Spider.

The elusive members of the genus Loxosceles are generally known as Recluse Spiders. They are sometimes called Fiddle-Back Spiders, Violin Spiders or Reaper Spiders. They are best known for their arachnidism that is far more powerful than their Six-eyed Sand Spider cousins. It kills the cells in living tissue causing gangrene (which could lead to amputation or death).

Recluse spiders are founded in the southern half of the United States (from coast to coast), Mexico and Hawaii.

Wandering Spiders

The genus Phoneutria (Greek for "murderess") includes the Wandering Spiders which are very dangerous due their aggressiveness. They are, as a family, considered the most highly venomous spiders in the world. The most dangerous of the family are the Brazilian Wandering Spider and the Brazilian Huntsman Spider.

Anyone having being bitten by a Wandering Spider must seek immediate emergency medical assistance because the bite is considered deadly.

Widow Spiders

...
A female Black Widow Spider.

The Black Widow Spider is the most infamous member of the genus Latrodectus. Other members include the Australian Redback Spider, the Brown Widow Spider, the Katipo Spider, the Red Widow Spider and the Button Spider. All of them can be find in urban and agricultural areas throughout the world, and they are considered very venomous.

There are several different variations of Black Widow Spiders that have arachnidism that is up to 15 times more dangerous than a rattlesnake bite.

Wolf Spiders

...
A Wolf Spider.

The genus Lycosidae (Greek for "wolf") is the family of spiders that is known to actually have hunting methods that resemble a pack of wolves. They are located throughout the world.

Yellow Sac Spiders

Yellow Sac Spiders, which are also known as Black-footed Spiders, make up the genus Cheiracanthium. Yellow Sac Spiders are known worldwide for their extremely powerful fangs and their bites are sometimes mistaken for those of the Brown Recluse Spiders.