What is the name of the dinosaur that had plates on its back and spikes on its tail?

The plates were not sharp, per say, but I believe you're referring to Stegosaurus and its relatives. They had two parallel rows of large, bony plates on their backs. The plates were vertical, rather than lying flat on the back like paleontologists originally believed. They were also hexagonal in shape. Stegosaurus and its relatives were quadrupedal herbivores, whose small heads were low to the ground as a result of having much longer hind limbs than forelimbs.