There are only two main types of poisonous lizards in the world that are widely accepted to fall into the poisonous lizards category. These are from the same family: Helodermatidae. The Gila monster, Heloderma suspectum, and the beaded lizard, Heloderma horridum, both carry venomous saliva in their gland ducts. Within these lizard species, there are also subspecies, but monitor lizards and komodo dragons are not widely included as poisonous and are in a different family.
Some studies have shown there to be a possibility that at least the komodo dragon does have some venom in its saliva. Even before the studies though, many people have mistaken the monitor lizard, Varanus, and the komodo dragon monitor species Varanus komodoensis, from the family Varanidae, as poisonous. Part of the confusion often has to do with the toxic bacteria that monitor lizards have in their saliva. The komodo dragon's saliva especially can cause death in humans from a secondary infection if the bite isn't treated promptly.
The Gila monster and the beaded lizard, on the other hand, have a neurotoxic venom rather than bacteria. This difference in the types of saliva between Varanus and Heloderma means that usually only beaded and Gilas are widely considered to be classified as poisonous lizards. Yet, the newer studies may change science's classification of at least the komodo dragon to a poisonous lizard if enough research shows there to be venom along with bacteria in the saliva of this species after all.
To further confuse the issue, even though Gilas and beaded lizards are poisonous, their mildly neurotoxic venom isn't known to kill humans, whereas, unless medical help is received, a bite from a komodo dragon typically results in death from infection caused by bacteria. Also, komodo dragons are the world's largest lizard and may weigh 300 pounds (136 kg) as well as grow up to 12 feet (3.66 m) in length. Contrastingly, beaded lizards are only about 1/6 to 1/4 of the komodo's size at around 2 feet (61 cm) to 3 feet (91 cm) long, while Gila monsters, despite their name, are even smaller and may only reach 1 foot (30.5 cm) in length. Although the venom that beaded lizards and Gila monsters carry is similar to that of the diamondback rattlesnake, it's not considered as dangerous due to the clamping and chewing release of it rather than more directly by the snake's fangs. The Gila monster's forceful, hanging-on type of bite is considered to be especially painful though.
Gila monsters are found in the Southwestern United States, while beaded lizards are located mainly in Mexico and southern Guatemala. Both of these poisonous lizards have scales that are stud-like, or bead-like, rather than overlapping. In general, Gila monsters have more colorful markings than beaded lizards. Gilas are known for their mainly black background with patterned markings of yellow, orange and/or pink. One beaded lizard subspecies, Heloderma horridum alvarez, is usually solid black, while other "beadeds" often have some yellow patterns.