I did some research on insects and spiders.
I always thought spiders are insects, but it seems they really aren't.
Spiders are arachnids.
Arachnids and insects both belong to arthropods, but are actually 2 different classes.
What's interesting about insects, is that they belong to a subphylum (subgroup, if you prefer) known as hexapods, which is Latin for "six feet", so insects have six feet.
Which means that centipedes aren't insects either. They belong to myriapods, another subphylum of arthropods.
The more you know.