Some spiders are poisonous and others are not.
There is difference between poisonous and venomous. Almost all spiders are venomous (their toxins are proteins and work by being injected, not by being eaten); only two small families lack venom glands. The purpose of spider venom is to subdue the spider's prey, almost always insects. In brief, it's an insecticide. Spider venom does not exist to harm creatures, like humans, which are too large for spiders to eat, and in nearly all cases has little if any effect on humans. Of around 50,000 spider species known, only about 25 (1/20 of 1%) have venom capable of causing illness in humans, to a greater or lesser extent.