Dolphins are really smart, some of the smartest water creatures. They are usually highly interactive with people and very social to each other. Their language is still a mystery to us, but biologists managed to study their behavior. Several studies even proved that dolphins employ a universal “sono-pictorial” language of communication. (http://speakdolphin.com/ResearchItems.cfm?ID=20)
They do not have vocal chords, so they use blowholes muscles controlling to make noises. The sounds they make include whistles, squeaks and ultrasonic which are not heard by people. Dolphins can identify each other through these sounds; every dolphin having a unique whistle.
Special whistles are used then dolphins are in need. Their pod usually answers quickly.
Dolphins do communicate by making sounds but not only. Scientists believe that dolphins communicate through body language: body posturing, jaw popping, jumping and falling back in the water with a splash, tail slaps.
It seems like they not only talk but also move to communicate different things :). Need to talk in many ways.
In the below movie you could see how dolphins play and communicate:
Hope you will enjoy it :).