Spiders and their relatives, such as scorpions, harvestmen, ticks, mites, and solifuges, all belong to the class Arachnida. All of these joint-legged invertebrates (meaning that they don't have a backbone) have four pairs of legs (with exceptions) and consist of two body segments, the abdomen and the cephalothorax, a fusion of the head and thorax. In addition to the eight legs, they have two pairs appendages specialized for different purposes. The first pair, called the chelicerae, serve for feeding and defense. The second pair are called pedipalps, which are use for feeding, locomotion, and sometimes reproductive purposes. Arachnids also lack both antennae and wings, which most clearly distinguishes them from insects.
Spiders are air-breathing arthropods with chelicerae with fangs that inject venom, though some have lost their fangs. They also possess appendages in their abdomens called spinnerets, which they use to spin silk, although not all spiders do so. They belong to the order Araneae, the largest order of the arachnids and they also rank seventh in total species diversity amongst the other orders. All known species of spider are predatory, with exception of Bagheera kiplingi, which is herbivorous.
Although all spiders are capable of producing venom, most of them do not possess fangs long enough to pierce our skin, and those that have fangs that are long enough, their venom is often not potent enough to kill us. The spiders that you truly have to watch out for include the Brown Recluse spider, Widow spiders, Hobo spider, Mouse spiders, Funnel-Web spider, White-Tail spider, and Black House spider. In fact some spiders are very helpful killing off unwanted pests. Such an example is the spider Heteropoda venatoria. This spider, growing to a size of 3 to 4 inches wide (legs included), actually prey on the infamous cockroaches and other household pests. The truth is that spiders enter a house either by accident or because there is a viable food source found inside of the household. Spiders don't stay in a place where they will starve to death.
Heteropoda venatoria size comparison