Dolphins are a general term for a group of aquatic mammals in the family of Delphinidae, which includes mainly small or medium-sized toothed whales.

They are widely distributed in the world's oceans, and can also be found in brackish water near inland seas and river estuaries, with some species, even inhabiting inland rivers. Dolphins usually live in groups and feed on fish, squid, and other prey.

Most dolphins have a sleek, streamlined body with a hooked dorsal fin, though some may have other forms as well. Some dolphins have striking and colorful patterns on their bodies, while others have more dull colors.

As marine animals, dolphins breathe on the surface of the water. They can hold their breath underwater for 20 to 30 minutes before coming up to breathe. When dolphins are seen jumping out of the water, it is because they need to breathe.

Dolphins typically live in shallow seas and are rarely found in deep waters. They engage in different activities in different areas. When resting or playing, they often gather in bays near the beach. When hunting, they are found in shallow water and rocky areas.

Like other marine mammals, dolphins do not drink seawater. Instead, they obtain water from their food to replenish what their body loses. The salinity of the blood in dolphins is about one-third that of seawater.

Dolphins are fast swimmers, with the fastest species reaching speeds of up to 55km/h and being able to maintain these speeds for a long time. They are champions of long-distance swimming in the ocean.

Dolphins have the most developed brains of any animal apart from humans, accounting for 1.7% of their body weight.

They normally sleep with alternate hemispheric brain activity, with one hemisphere inhibited while the other is excited. If encountering strong external stimuli, dolphins can quickly wake up to ensure their mobility and cope with emergencies.

Some dolphins are highly social species that live in large groups, sometimes consisting of more than 100,000 individuals.

They exhibit many interesting collective behaviors, including cooperation in a variety of ways such as attacking sharks and rescuing injured or sick individuals. Dolphins often follow boats and ride the waves, jumping into the air like acrobats and creating a spectacular sight.

Dolphins are very intelligent and have complicated grooved brains and good memory. They can learn many actions under human training and have one of the highest IQs among animals. Dolphins have a gentle, friendly, lively, and active personalities and are loved by people all over the world.

Interestingly, dolphins have DNA that is similar to humans, with almost identical chromosomes. The main difference between a dolphin's genes and human's is the way the chromosomes are arranged.

They are also self-aware and are one of the few animals that can recognize themselves in a mirror. When a dolphin sees a mirror for the first time, it may interact with its reflection by playing with or blowing bubbles. They also use the mirror to check their grooming.

Many cultures around the world have long-standing myths and legends about dolphins. In Greek mythology, dolphins were associated with the god of the sea, Poseidon, and were often depicted as helpers of sailors in distress.

In some Native American cultures, dolphins were considered to be powerful healers and were believed to have the ability to cure people of various ailments.

In recent years, there has been controversy surrounding the use of dolphins in captivity for entertainment purposes, such as in dolphin shows at theme parks.

Some argue that these practices are cruel and harmful to the animals, while others argue that they can be educational and help promote conservation efforts. This is a complex issue with passionate opinions on both sides.