what are the characteristics of a whale?? and Why is it considered as mammals?

Whales are generally predators, although their prey could be microscopic to very, very large. They communicate with each other using loud sounds that can be heard for miles. Their body represents that of a fish, they have flippers and fins to help them move through the water. Beneath the skin, they have a layer of fat or blubber. Whales can usually live for a long time, 40-200 years.

They are considered mammals becuase they are warm blooded and need oxygen to live. They get this oxygen by coming up to the surface and breathing through their blowhole, although they can hold their breath for quiet a long time underwater.

Whales are large sea going mammals (cetaceans). Some are herbivore herd animals (a pod is the whale equivalent of a herd) some are solitary carnivores. All are air-breathing despite spending there whole lives in water. There is a very wide range of size from 2 meters to almost 30 meters long, most have considerable amounts of body fat, all have nostrils that are above and behind their eyes. Because they have very different ways of eating they have a wide range of body types. But all have flukes that are horizontal unlike fish that have tails which are vertical.

They are mammals because they have hair and nurse their young, the 2 main criteria for being classified a mammal.

It is a mammal because it breathes air and gives birth to live young and they suckle.

You can find all of the answers to your question at Wikipedia. It gives all of the reasons that whales are mammals.

http://en.wikipedia.org/wiki/Whale