People unfamiliar with the hobby of tarantula keeping often assume that such large spiders pose a significant danger to a human bitten by one. However, though the assumption seems logical, the reality is a bit different.
It is true that all tarantulas, in fact all spiders, are venomous. But only a very few species of tarantula posses venom that poses any kind of threat to humans, and there is no verified case of human death by tarantula bite. The majority of tarantulas found in the pet trade are wild-caught, so there is no difference between wild and "domestic" tarantulas. That said, pet tarantula bites are quite rare, as are bites from wild tarantulas. Tarantulas in general are reluctant biters; given the choice between biting or retreating, the vast majority will choose retreat every time.
Tarantulas can be broken into two generalized groups; old world and new world species. It is among the old world species, particularly those from Africa and India, that the more serious venom is found. Unfortunately these species rely on their painful venom as a means of defense and so are more prone to biting when they feel threatened. Luckily for beginners, these species are usually available only through hobbyists or importers who mark the species as potentially dangerous, and are very rarely found at the local pet store.
New world species have venom that is very weak to humans, but they rely on a different defense- urticating hairs. When threatened, new world species, rather than biting, will use their back legs to flick a cloud of hairs from the top of their abdomen. These hairs are barbed and can produce a maddening itch that lasts for days- particularly if the hairs get in the mouth or eyes. The hairs, once shed, will not regrow until the spider molts; some tarantulas have completely bald abdomens as a result of vigorous hair flicking.
For most people, keeping pet tarantulas is rather like keeping fish- you look but don't try to interact. Tarantulas tend to be very solitary animals and have no desire to socialize with their keepers or any other creature, including other tarantulas. General care, such as feeding, changing water or cage cleaning, is very unlikely to provoke the animal into biting or hair flicking.
This is not to say that people never handle their pet tarantulas- to the contrary, an experienced handler can pick up even the most flighty or defensive species with relative ease. The majority of pet tarantula bites seem to be the result of hasty attempts to capture escaped animals. Confident, calm handling seldom results in a bite.
It is important to note that tarantula venom, like all venom, can affect different species of animals differently. So while one can confidently claim that no new world species of tarantula is dangerous to humans, the same might not be true of other pets- a particular type of venom might be fatal to pet cats, dogs, rabbits and so on. Of course it is also possible that it is NOT dangerous to these other animals; there is no master list of the effects of different types of tarantula venom on different species of animals. In fact surprisingly little is known about the subject at all; scientists have barely scratched the surface investigating properties and possible medical uses of tarantula venom.
Pet tarantulas can be fascinating to watch, are very inexpensive and easy to care for, and are not messy or noisy like many other pets. Some species are thought to live as long as 60 years, certainly not a commitment to make lightly!