Sauropoda (meaning "lizard feet") was an infraorder of Dinosaurs that Longnecks are classified as. It is in the order Saurischia and suborder Sauropodomorpha.
Sauropods were huge quadrupedal plant eaters. All had extremely long tails and necks. For a long time the necks were thought to be used to reach high-up leaves but later it was discovered that many species could not move their heads far above the hight of their backs. The long tails were mainly used for balance and defence. Overall they have the same sort of general look.
Species appearing in The Land Before Time: stub
Read More at: Sauropod on Wikipedia