A spider is an eight-legged critter that eats other critters. Where the term ‘spineless’ is considered a bit of an insult for humans, for spiders it would be perfectly normal because they don’t have a spine at all. Instead they carry their squishy bits around inside a hard armour-like skin called an exoskeleton.
Now, apart from the bit about having eight legs I could have been describing a lot of other critters including several insects here, so I’ll now mention some more things that define them as spiders.

You'll see in the cartoony pictures above that the spiders doesn't have wings. That means spiders can't fly, although some of them do manage tricks which allow them to either glide or be carried by the wind. You'll also see that the insect has its body divided into three main bits (head, thorax and abdomen) while the spider has only two bits. That's because the spider's head and thorax are combined into one section called a cephalothorax.

And here's another thing about spiders: they're not able to chew their food.

Outgrowing their skins
I mentioned earlier that spiders wear a tough skin called an exoskeleton to hold their soft bits together. That system works well for spiders but it has its limitations too. The exoskeleton is not able to keep expanding as the spider inside it gets bigger. So as a spider grows up it sometimes has to produce another skin and shed the old one. The picture below left shows the discarded skin of a huntsman spider.

Discarded spider skins are more interesting than most people realise. If you look at the close-up picture alongside it you’ll see that it’s covered with lots of stuff like hairs and spines. So the spider sheds a lot of stuff with that skin. Of course I was careful to see that the spider was not still in the skin when I picked it up. When you see the size of its fangs you'll get a better idea why.