Albertosaurus was a mid-sized theropod that flourished throughout what is now North America during the Campanian era of the late Cretacious about 75 million years ago.  It can best be described as a smaller, more lightly built version of its later, more famous relative, Tyrannosaurus rex.  It coexisted with and most likely hunted other famous dinosaurs like Parasaurolophus, Styracosaurus and Pachyrhinosaurus just to name a few.