In terms of iconic dinosaurs, the gargantuan sauropods are certainly up there. Along with the mostly meat eating-theropods, and herbivorous and often armoured ornithischians, they form one of the three major groups, or clades, of dinosaurs, and were the biggest animals to ever walk this Earth.
The end of the Jurassic period, some 145 million years ago, was a pretty important time for sauropods. Their diversity was already in decline through some of the latter part of the Jurassic, but it seems that they were hit pretty badly at the Jurassic/Cretaceous (J/K) boundary, in an extinction event that may have been quite severe among land and marine-dwelling animals.