Spiders eat small insects, plant nectar as well as other spiders. The larger varieties eat small birds and lizards while there is a vegetarian species that feeds on acacia trees. There are more than 40,000 different species of spiders in the world.
Spiders eat different things depending on their species. Most of the spiders eat small insects and other spiders that get trapped in their spider web. You can find more information here: http://wanttoknowit.com/what-do-spiders-eat/
Spiders will eat other spiders, insects, invertebrates and some small vertebrates. Spiders do not eat plants. They are predators and only eat living animals. Some will inject venom into their prey if it is large to immobilize it.