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The black widow spider is a well-known member of the genus Latrodectus. The female black widow's venom is particularly harmful to humans (males almost never bite humans) and the injection of venom from these species is a comparatively dangerous or lethal spider bite, resulting in the condition Latrodectism which is named for the genus.

In common with other members of the Theridiidae family, widow spiders construct a cobweb, i.e., an irregular tangle of sticky silken fibers in place the common depiction of a spiderweb. The black widow spider very frequently hangs upside down near the center of its web and waits there for insects to blunder in and get stuck. Then, before the insect can extricate itself, the spider rushes over to bite and swathe it in a silken shroud. If the spider feels threatened it will normally let itself down to the ground on a safety line of silk just as fast as it can. As with other web-weavers, however, these spiders have very poor eyesight and depend mostly on vibrations reaching them through their webs to orient themselves to prey or warn them of larger animals that could injure or kill them. They are not aggressive, and most injuries to humans are due to defensive bites delivered when a spider gets unintentionally squeezed or pinched somehow. It is possible that some bites may result when a spider mistakes a finger thrust into its web for its normal prey, or in cases where a female is protecting an egg sac, but ordinarily intrusion by any large creature will cause these spiders to flee.

Although these spiders are not especially large, their neurotoxic venom is extremely potent. Compared to many other species of spiders, their chelicerae are not very large or powerful. In the case of a mature female, the hollow, needle shaped part of each chelicera - the part that penetrates the skin - is approximately 1.0 millimeters (about 0.04 in) long, long enough to inject the venom to a point where it can be harmful. The actual amount injected, even by a mature female, is very small in physical volume. When this small amount of venom is diffused throughout the body of a healthy, mature human it usually does not amount to a fatal dose (though it can produce the very unpleasant symptoms of latrodectism). Deaths in healthy adults from Latrodectus bites are relatively rare in terms of the number of bites per thousand people. Sixty-three deaths were reported in the United States between 1950 and 1959. Widow spiders have more potent venom than most spiders, and prior to the development of antivenin, 5% of reported bites resulted in fatalities.