Black widows are spiders. Their scientific name is Latrodectus mactans. They share the genus name Latrodectus with several similar spiders (sometimes called "widow spiders").
The female spider is shiny black and has a round abdomen with a red hourglass pattern on its underside. They are closely related to another group called Steatoda (sometimes called "false widow spiders). That group contains species that are not dangerous to humans. All of the true widow spiders can give bites that are harmful to human beings and may kill children or people who are not in good health.
Female widow spiders are rather large. They have a body length of approximately 0.5 inch. The males of the widow spiders are much smaller than the females.