Whales are mammals, just like you (and Otto for that matter). This means that despite how big they can get and that they live in the ocean, you have a lot in common with whales! Because they are mammals (not fish!), whales give birth to live young, nurse their babies, and are warm blooded! There are two different main categories of whale - baleen and toothed, and they all fall under one umbrella term: cetaceans. Even though they are all part of the whale family, people often consider dolphins and porpoises different animals. In reality, they are what's called toothed whales. Many of the Great Whales - like Blue Whales and Humpbacks - are Baleen Whales, except for the Sperm Whale which is toothed. The Great Whales are called "great" because of their great, big size!
So, what's the difference between toothed whales and baleen whales? It has to do with what's in their mouth! Toothed whales have, well... teeth! They usually hunt and eat fish, squid, crabs, and other ocean animals. Baleen whales are called filter feeders. They take massive amounts of water into their mouths and use their baleen plates (which have a lot of hair like structures on them) to filter out food like krill and plankton, as they push the water out!
The Blue Whale is the biggest of the whales. In fact, the Blue Whale is the biggest animal EVER - even bigger than the dinosaurs! It's tongue alone can weigh the same as an elephant! Can you imagine having a tongue that big?!