Tapioka is Manihot esculenta, used for its tubers. They are called cassava or yucca roots as well. In Indian cuisine they are used for instance boiled or mashed like potatoes, with turmeric added as a natural colorant.
Tubers are used for starch as well, which is part of vietnamese steamed rice panacekes for instance or as a thickening agent in many kinds of dishes.
Tapioca has great nutritional profile and is used often as a part of celiac diet.