They all 'true' seals, in that they cannot use their back legs the way, say, a Sea Lion or a Fur Seal can. I was stationed on the Macquarie Island north of Antarctica for a full year and saw many different types of seal, including the giant Sea Elephants - also a 'true' seal.
Hollywood has painted the Leopard Seal quite inaccurately. On the beach or ice they are slow, rather sluggish and not aggressive. It is in the sea that they become the marauding predator, their main prey being penguins.