All spiders have two body sections: the cephalothorax in front and an abdomen behind. The abdomen contains the digestive and reproductive systems, and on the underside of it are the glands where silk is produced. The structures that produce the silk are called spinnerets.
They have eight legs, all attached to the cephalothorax. On the front of the cephalothorax are the mouth, the fangs, the eyes, and two small "mini-legs" called pedipalps. These are used to grab prey, and in mating, and are much bigger in male spiders than in females. Different species of spiders have six or eight eyes, and the size and arrangement of eyes is different in different groups. All spiders have fangs that they use to bite their prey with, and most have venom glands.
Female spiders are often much bigger than males.
Spiders are everywhere! Baby spiders are so light, they can put out a line of silk and float away on a breeze, and so they are spread around the globe. Also, spiders that live in peoples' houses often get moved around by accident when people move. Spiders are found on every continent, and there are over 40,000 species known to science. That's not all of them though, there are thousands more we don't know about yet.
Spiders survive in every habitat on land except the very coldest. There are even a few in shallow fresh water.
Spiders hatch from eggs, and the hatchlings look more or less like grown-up spiders, though sometimes their colors change as they age. To grow they have to shed their exoskeleton, which they do many times during their lives. Unlike insects, some spider species keep growing after they become adults, and continue to molt as they get bigger.
After mating adult female spiders lay dozens to hundreds of eggs. The number varies among species and is affected by how well fed she is.
Female spiders often guard their eggs and young, or even carry them with them until the young have hatched and molted.
Most spider species live a year or less, but some large ones can live for several years. Usually males die soon after they mate, it's only the females that can live longer.
Nearly all spiders live as solitary hunters, only finding another of their species to mate. Many species make complicated webs to trap prey and give themselves protection. A few tropical species live in groups in really big webs.
Spiders are very sensitive to vibrations, especially in their webs. Most don't see very well, but the Jumping Spiders see very well.
All spiders are predators. They attack only live insects, other spiders, and other invertebrates. A few very large spider species attack small vertebrates like lizards, minnows, or frogs, but this is rare. Spiders are famous for trapping their prey in webs of sticky silk, but many of them are wandering predators who don't use silk to catch prey.
Spiders often have good camouflage, and many of the wandering hunter species hunt at night when big predators can't see them. Many spiders build a retreat of silk to hide in too. Some are quick runners, and will just run away if they can. These may use their silk as a safety line, jumping off into the air with a silk thread attached so they don't fall.
Spiders are major predators of insects, including many insects that are pests to humans.
Spiders scare a lot of people (usually this is unnecessary). A few spider species have venom that is dangerous to people, but spider bites are actually pretty rare. They get blamed for a lot of skin injuries that are not actually spider bites. Most spiders have such small weak fangs that they couldn't break a person's skin, even if they wanted to.
Spiders eat a lot of insects that are agricultural pests and eat our food.
George Hammond (author), Animal Diversity Web.