I agree with pokepokehmm...
Insects have a head, thorax, and abodmen, and six legs coming from the thorax
Spiders have a cephalothorax and an abdomen, with eight legs coming from the cephalothorax... All spiders are arachnids, but not all arachnids are spiders... ie. scorpions, vinigaroons, camel spider, mites, etc...
If you use the term bug as something more than a laymen's term, you are reffering to the order Hemiptera, which is in class insecta...
Bug in laymens terms is pretty much any arthropod...