Which of these is a tetrapod: a spider, elephant or goldfish?
If you're thinking elephant, you are correct! The prefix "tetra" means four, and "pod" is Greek for feet.
Tetrapods are vertebrates with limbs. Spiders don't have backbones, so they aren't vertebrates. Fish are vertebrates, but don't have limbs. Before about 360 million years ago, no vertebrates had limbs. They all shared the same fish-like body shape. No limbs, just fins.
Around that time something exciting happened. The fleshy lobed paired fins of some prehistoric fish evolved into limbs. The modified fins gradually evolved into legs capable of supporting the animal's weight, and allowing it to walk out of the sea. Tetrapods evolved not only limbs, but also digits.
One false notion used in attempts to discredit the theory of evolution, is that there aren't any so-called "transitional fossils." In other words, there aren't examples of intermediate forms between two branches on the evolutionary tree, such as between prehistoric fish and tetrapods.
But a particularly exciting discovery was made in 2008. Scientists used CT scans to look at the bones hidden inside the intact fossilized pectoral fins of a 385-million year old lobe-finned fish discovered in Latvia. Inside the fish's stumpy limb-like fins were rudimentary digits, like a tetrapod. This fossil is just one of many that illustrates the gradual evolution of legs that allowed early tetrapods to leave the water and colonize the land.