A squid's diet consists of shrimp, plankton, crabs, and small plants. Squid also eat small fish. Larger squid will sometimes feed on smaller squid, as well.
1 Additional Answer
There are 300 known species of squid and their diets vary somewhat based on their size. All squid are carnivores. They've been known to eat many animals including crustaceans, fish and even other squid. You can find more information here: http://www.buzzle.com/articles/what-do-squid-eat.html