Apatosaurus was called Brontosaurus until it was discovered that the two animals were, in fact, the same. Paleontologist O. C. Marsh named a specimen he uncovered in 1877 Apatosaurus ajax. Marsh discovered parts of a second fossil skeleton in 1879 and, not recognizing it as the same animal, called it Brontosaurus excelsius. Since the name Apatosaurus was published first, it takes priority as the valid name. – The Columbia Electronic Encyclopedia, Sixth Edition
It's also interesting that we had the wrong skull on Apatosaurus for roughly 100 years. It threw off the accuracy of old illustrations and is still making it's rounds—just do a search for Brontosaurus images. For more about Apatosaurus, Casey from Kapunahala Elementary can fill you in. Be sure to check out his classmates' reports as well—they've seriously been the best reports I've seen in a long time.