Tarantulas are large, hairy, and vicious looking spiders that are common throughout much of the southern part of the US, southward into Mexico, Central America, and South America, and in the tropics and subtropics of other parts of the world. They are yet another member of the arthropod subgroup "chelicerates"; in tarantulas, the chelicerae are developed into large fangs. The pedipalps are long and palp-like and presumably are used for locating and manipulating prey items. Tarantulas are venomous, and although they rarely bite humans, humans handle them often enough (because we keep them as pets) that occasional bites are reported. The bite of a tarantula is said to be painful but not dangerous unless you are allergic to the venom, in which case it can cause a systemic allergic reaction called "anaphylactic shock". If you are bitten by a tarantula and begin to experience tightness in the chest and shortness of breath, going to a hospital emergency room would be a good idea.
Tarantulas are relatively "primitive" spiders, which doesn't mean they are not well-adapted for life in their chosen habitat, but simply means that they possess many characteristics that would have been present in the ancestral spider. For instance, tarantulas are nocturnal and burrowing, and use silk only for lining the burrow and not for prey capture (as the yellow garden spider does). They have relatively poor eyesight, especially compared to jumping spiders and other visually orienting predaceous spiders. Tarantulas hunt at night for small arthropods and other invertebrates, and will occasionally capture small vertebrates. Their prey-location behaviors are also somewhat primitive; some tarantula species hunt by simply blundering about in the habitat, grabbing and biting whatever they encounter. Other species probably have more sophisticated hunting techniques, but I have found no information about that.
What little information I have found about the life cycle of tarantulas in the wild indicates that they spend the winter (if they are in sub-tropical or temperate regions) in their burrow, and begin to emerge in the spring to forage and search for mates. Males typically roam farther than females, presumably because they are doing the searching. I have no details on courtship and copulation, but more advanced spiders have elaborate courtship rituals.
You would think that animals as large and creepy as tarantulas would be well-known to science, if only because scientists (biologists especially) seem to be attracted to strange and potentially dangerous natural phenomena. However, new species of tarantulas still turn up every so often. By "new species" I don't mean ones that have just diverged from an ancestral species, but species that have not been described in the formal fashion that taxonomists require when new species are found. I recently reviewed two separate scientific manuscripts describing new tarantula species, two from southwestern Mexico and one from Belize. About 1500 tarantula species have been described, but obviously we haven't found them all yet. Someone needs to be out there looking!
Tarantulas are of course predators, but they also have their own predators.
The most significant of these are tarantula hawk wasps, which as you might
expect are large. Tarantula hawks fly around slowly, a few meters above
the ground surface, scanning the ground below for wandering tarantulas.
The few times I have observed encounters between the two species, the interaction
was dramatic to say the least. The tarantula would rear up with the pedipalps
and fangs at the ready, but the wasp typically was quicker and more agile,
plus the wasp can fly. The usual outcome was that the spider was stung
by the wasp, who would then carry the spider away to a burrow (with great
difficulty), where the wasp would lay an egg on the tarantula. The egg
hatches into a wasp larva, which is a "parasitoid", meaning it lives in
the body of the tarantula (they are paralyzed but not killed by the sting),
feeding on the internal organs, saving the vital organs for last. Eventually
the wasp larva pupates (still inside the tarantula), and when it ecloses
the wasp breaks out of the now dead tarantula's exoskeleton, like an alien
jumping out of your chest.