Size and Weight:
If it weren't for one tell-all feature, Spinosaurus might have been indistinguishable from any other large theropod stalking the swamps of the Cretaceous period. That feature, of course, was the extensive, fin-shaped sail on its back, a thin flap of skin supported by sharp needles of bone that protruded from Spinosaurus' vertebrae. (See 10 Facts About Spinosaurus and a gallery of Spinosaurus pictures. Also, here's an article about the discovery and naming of Spinosaurus.)
Why did Spinosaurus have this strange-looking sail? The most likely explanation is that this structure evolved for cooling purposes in the hot northern African climate in which Spinosaurus lived (a bit like the big, floppy ears of African elephants). It may also, as a byproduct, have been a sexually selected characteristic--perhaps male Spinosaurus adults with bigger sails had more success mating with females. (See Why Did Spinosaurus Have a Sail? for a deeper analysis of this anatomical puzle.)
By the way, paleontologists now believe that Spinosaurus was the largest carnivorous dinosaur that ever lived--outclassing even Tyrannosaurus Rex by one or two tons. Fortunately--or unfortunately, if you happen to be a movie producer--these two dinosaurs didn't share the same time or territory, T. Rex living tens of millions of years later in North America, not Africa. Spinosaurus did, however, share its territory with the 10-ton prehistoric crocodile Sarcosuchus; see Spinosaurus vs. Sarcosuchus - Who Wins? for an analysis of this epic battle.