Spiny describes an animal or plant covered with stiff sharp points — things that would hurt to grab. A cactus is spiny. A hedgehog is spiny. So are sea urchins, some fish, and bushes full of thorns. The points can be thorns on a plant or hard needle-like spines on an animal; either way the word says 'sharp, stiff, and lots of them'. It is a normal everyday word, though you meet it most in nature writing and in animal names, where it does real work: spiny lobster, spiny anteater, spiny mouse — the name tells you what the creature feels like. It is nearly always used about living things. For a sharp man-made object you would say 'sharp' or 'pointed', not 'spiny'. Note also that spiny points are stiff and thick, unlike 'hairy' or 'furry', which are soft.