Growing up, I assumed that eight legs were a part of classifying spiders, but that is not the case. Many other little creatures have eight legs, e.g., ticks. So what then makes a spider a spider? For the right and correct answer, it is the following:

“The most important synapomorphies of the Araneae (which would indicate their monophyly) are the following (Coddington, 2005; Coddington and Levi, 1991): (l) abdominal extremities transformed into spinnerets; (2) abdominally located spinning glands; (3) chelicerae with venom glands; (4) male palp modified for sperm transfer; and (5) loss of abdominal segmentation” (1:335)

If an arthropod has these characteristics, it is classified as a spider. Now you know. Many websites said other things, but they are not correct, these characteristics make up the essence of a spider.