This is a common confusion, actually spider is not an insect.
What is an insects? Insects are six legged animal. All insects have two feelers, called antennae, on their heads, and most of them have wings. Insects have three main body parts - the head, the thorax, and the abdomen. Most important is they have three pairs of legs.
But, spiders have four pairs of legs and have no feelers and wings. Their bodies are divided into two main parts the abdomen and the cephalothorax, where the head and the thorax are joined together.
Scientists classify spiders as Arachnids.
Arachnids have one to six pairs of simple eyes and some species are eyeless. But, insects have many small eyes crowded together called compound eyes. Other arachnids are Scorpion, ticks and mite.