The California roll is a maki-zushi (roll), a kind of sushi roll, usually made inside-out, containing cucumber, crab meat or imitation crab stick, and avocado. In some countries it is made with mango instead of avocado. Sometimes crab salad is substituted for the crab stick, and often the outer layer of rice (in an inside-out roll) is sprinkled with toasted sesame seeds or tobiko. Read more on Wikipedia.
Watch a video on how to make an inside out California Roll.