The full name of this Sauropod from the late Jurassic period is Diplodocus Carnegii. Diplodocus means "Double Beam", a name given due to the forkings on the animal's vertebrae *.
Diplodocus is thought to have been the longest of the sauropods, measuring up to 45 metres from head to tail, with a lot of the length taken up by an extremely long, bony and flexible tail, which paleontologists think may have been used as a defensive, whip-like weapon against predators. It may have weighed up to 30 tonnes. It had a brain the size of an apple, but, like many sauropods, somewhat made up for this by the handy expedient of a second brain, used to help with motor control, at the base of its spine.
Like Brachiosaurus, Diplodocus was once thought to be a slow-moving, marsh-dwelling creature so massive that its lizard-like metabolism could hardly cope with its own weight. More recent thinking on dinosaurs suggests that, being warm-blooded, Diplodocus may have been more mobile and have had a larger choice of habitat. Where for Brachiosaurus you should be thinking giraffe, for Diplodocus, think cow. Enormous, extremely stupid cow.
* - thanks to Kyberneticist and Google for this.