Dolphins use several methods to get their food. Fish and squid are the main sources of food for most dolphin species, and they hunt in group cooperatively to catch them.
One method called herding consist that a group of dolphins encircles a school of fish to gather them as close as possible and after that one or two take turns to swim from the bottom of the school to have the largest amount of fish possible on each pass.
It is important that they go from bottom to top because if they were from side to side, the school of fish would probably disperse.
The second method is called corralling, which is performed taking fish to shallow waters where dolphins can just capture them.
Some species are even more specialized or with unique techniques to catch fish or other particular prey that only they consume, like killer whales hunting seals, for example.