The little balls in tapioca pudding are pearl tapioca. Tapioca is a starch from the root of the cassava plant. This plant contains toxins and requires processing before consumption.
Manufacturers process tapioca in several forms of starch, including powder, flakes, sticks and pearls. The sticks and pearls require soaking in water before use. Upon soaking, these forms double in size and take on a leathery texture. Asian cooking frequently uses tapioca.
Tapioca balls are available in sizes ranging from 1 millimeter to 5 millimeters, with small tapioca the most common in the United States. Another common use for tapioca is in bubble tea. The boiled pearls add a contrast to the sweet flavor of the drink.