Additionally, each group contains young males and sometimes one, occasionally two, adult males. Adult males are much larger than adult females so large animals are most always males. So whales are mammals and not fish. Because of that, the most important ways they differ from fish have to do with the fact that fish evolution all took place in the water, while much of the evolution of whales took place on land. This means that whales have a lot of traits and behaviors that land mammals have and that fish don't have.
Whales are airbreathers with lungs while fish breathe with gills.
If you replace the hind limbs of a land mammal with a whale tail, the arching backbone that once sent a cheetah across an African plain can also send a fin whale through the ocean. In general, killer whales are more common close to shore and in fairly cold waters. They are less common in the tropics and open ocean, where we are spending most of our time, but you can see killer whales in all waters of all the oceans. I'll tell you why. Evolution happens at such a slow pace that humans can't really percieve it so we have to look for clues.