The name Black widow spider is commonly used for several different spider species within the genus Latrodectus. The genus contains 31 recognized species. In North America, the name Black widow spider is chiefly used for three different spiders: The Northern black widow spider (Latrodectus variolus), Southern black widow spider (Latrodectus mactans) and the Western black widow spider (Latrodectus hesperus).
                                                                                                                   
In Australia, the Red-back spider (Latrodectus hasselti) is sometimes referred to as Black widow spider. The name Black widow spider is also commonly used for the Grey widow spider (Latrodectus geometricus) that lives in Australia, South Africa and the southern United States, as well as for the Red Widow Spider (Latrodectus bishopi) which is endemic to a few parts of Florida.

The adult female Black widow spider has a characteristic hourglass shaped marking on the bottom of her abdomen. (In the Northern black widow spider, the marking is sometimes separated into two non-connecting patches.) The hourglass shaped marking is usually red, but it can also be of various yellow and orange shades. Some spiders will instead display a pure white hourglass shaped marking. The main body of the female Black widow spider is shiny black and you can often see small dots close to the spinerettes. The dots are usually red, but can vary in colour just like the hourglass. Large female Black widow spiders will be around 1.5 inches (38 millimetres) in size, legspan included. The main body is no more than 0.5 inches (13 millimetres).  

A male Black widow spider will be considerably smaller than the female; around half her size or even smaller. His legs are longer and his abdomen is smaller compared to his body size. Most male Black widow spiders have a dark brown coloration and are decorated with stripes and dots. They lack the characteristic hourglass shaped marking.

If you are unsure whether you have found an adult male Black widow spider or an immature female, you can look at the shape of the body. Adult males will typically have a very slender body compared to the immature females. The male Black widow spider will also have longer legs and big pedipalps. Young Black widow spiders are white, and will eventually molt to dark brown or black. 

The juvenile female Black widow spider will have her brightly coloured marks on her back, while the adult female always display them on the front of her body. This has to do with their different lives. The bright colours serves as a warning to predators – don’t eat me I’m poisonous. This type of warning sign is not only displayed by spiders; they are used by a wide range of other creatures as well. Since the adult female Black widow spider spends her time in her net; hanging and moving upside down, she must display her colour warning on the front of her body where it will be seen by a majority of possible predators. The juvenile female Black widow spider on the other spends most of her time travelling around; searching for a suitable place to settle down. Since she is walking around on her legs, the colour warning is displayed on her back.     

Black Widow Spider Facts