Orcas (also called "killer whales") are one of the most recognizable cetacean species because of their distinctive black and white markings. They're also known for their large dorsal fin, which can reach more than five feet in an adult male and about three feet in females. Variations in dorsal fin size and shape, and in so-called saddle marks the gray patch behind the dorsal fin, are used by those studying orcas to tell them apart.

Orcas are the biggest member of the dolphin (Delphinidae) family. Adult males reach an average length of 23–27 feet, while females reach around 20–22. An adult orca's weight can be anywhere from 5,700 to 19,840 lbs (males are heavier then females). At birth, both males and females are around 6.5 to 8 feet long and about 395 lbs.

Orcas are called killer whales because they hunt and eat other cetaceans (even blue, gray, and sperm whales), seals, sea lions, sea birds, fish, and turtles. Their speed (the record for an orca is 34.5 miles an hour) and well-toothed (20 and 26 cone-shaped teeth distributed between top and bottom) jaws make them well suited to be predators. However, some orca populations are apparently entirely fish eaters: The orcas in Iceland appear to prefer herring; around Vancouver Island, the food of choice is salmon. Essentially they adjust their dietary habits to local ecology—this is known as having a "catholic" diet. Whatever they eat, orcas must consume about 5% of their body weight each day.

Orcas can be found in all the oceans of the world, though they are more often seen in cold waters. The worldwide population of orcas is unknown, but it is probably several tens of thousands. (The number often given is 100,000.) They are not endangered or threatened, although some stocks may be. The greatest threats to orca populations (in no particular order) are habitat loss, diminished food supply, capture for the public display industry, pollution (including noise), human predation, and boat traffic.

The best-studied populations of orcas are in Canada (British Columbia), the United States (Puget Sound and Alaska), Norway, and Patagonia. Some work has also been done in sub-Antarctic waters, the Antarctic, and off Siberia.

Orca populations studied to date appear to have three basic social structures—"resident" (possessing lifelong family bonds, living in large matrilineal groups, feeding mainly on fish, vocalizing in highly variable, complex "dialects"); "transients" (possessing more fluid, less persistent family bonds, living in smaller groups, feeding mainly on other marine mammals, vocalizing in less variable, less complex dialects); and "offshores" about which little is known.

The basic unit of resident orca society is a mother, all of her dependent offspring (approximately ten years or younger), and her adult offspring as well, including her sons. Females will eventually spend less time with their mothers, as they begin producing calves of their own, but resident males appear to remain with their mothers for their entire lives. They leave for short periods to mate outside of their maternal group, but return to their mother afterward.

In the wild, orcas have a relatively high mortality rate during infancy (perhaps as high as 50% before one year of age). However, if a calf survives to its first birthday, a male can expect an average life span of 30 years, while a female can expect an average life span of 50 years. The maximum estimated life span for male orcas in the wild is 50–60 years. The maximum estimated life span for female orcas in the wild is 70–80 years. In captivity, orcas often die before they reach 20 years of age—almost no males achieve 30, and only a few females have lived past 30.

There are approximately 50 orcas in captivity worldwide, in the United States, Canada, Japan, France, and Argentina.