Are all spiders venomous / poisonous?

Most Australian spiders have venom, but the overwhelming majority are considered harmless to humans as the venom usually only affects invertebrates. A group of small spiders in the family  Uloboridae lack venom glands completely, so are the only non-venomous spiders.  There is a distinction between venom and poison, which is often confused in regard to spiders. Spiders are considered venomous because venom is a chemical which is injected (via a sting or bite).  Poison on the other hand is absorbed or ingested, therefore spiders are not poisonous.