The Chordata is the animal phylum with which everyone is most intimately familiar, since it includes humans and other vertebrates. However, not all chordates are vertebrates.
All chordates have the following features at some point in their life (in the case of humans and many other vertebrates, these features may only be present in the embryo):
dorsal nerve cord - a bundle of nerve fibers which runs down the "back". It connects the brain with the lateral muscles and other organs.
notochord - cartilaginous rod running underneath, and supporting, the nerve cord.
post-anal tail - an extension of the body past the anal opening.