The basic answer is yes; all spiders and tarantulas are venomous (although, there are a few that are not). However, that does not mean they are all dangerous. There are about 40,000 species of spiders recorded, and only about 200 are considered medically significant to humans. That means only 0.5% of all the spiders in the world are potentially dangerous to people! The vast majority of spiders are safe and not dangerous to humans. For tarantulas, the news is even better; there has never been a recorded death from a tarantula bite ever. See our post for more information about this topic.
Spiders and tarantulas are categorized under the order Araneae. Anything under the Araneae order is considered a spider. One of the ways we classify spiders is they have venom glands (1:335); therefore, all spiders and tarantulas are venomous, but again, that does not mean they are all dangerous!
Many animals are venomous that are not dangerous to people. For example, honey-bees are venomous, but unless you are allergic (as with all venomous animals), you will not die from an envenomation; it would hurt a little bit or itch, but that is it. Another example of this would be fire-ants. So just because spiders and tarantulas are venomous, does not mean they are necessarily dangerous to humans. Most spider and tarantula venom will have little effect on a person.
If you believed that all spiders are dangerous to humans, I hope this helps change your view of spiders to the beautiful and interesting animals that they are.