Tapioca is an essentially flavourless starchy ingredient, or fecula, produced from treated and dried cassava (manioc) root and used in cooking. Cassava flour, also called tapioca flour or tapioca starch, can also replace wheat flour, and is so-used by some people with wheat allergies or coeliac disease. As cassava is a gluten-free natural starch, there have been increasing incidences of its appearance in Western cuisine.
But watch out, cassava roots and leaves cannot be consumed raw because they contain two cyanogenic glucosides, linamarin and lotaustralin. (http://en.wikipedia.org/wiki/Cassava) According to that article, “for some smaller-rooted ‘sweet’ varieties, cooking is sufficient to eliminate all toxicity. The cyanide is carried away in the processing water and the amounts produced in domestic consumption are too small to have environmental impact. The larger-rooted "bitter" varieties used for production of flour or starch must be processed to remove the cyanogenic glucosides. The large roots are peeled and then ground into flour, which is then soaked in water, squeezed dry several times, and toasted.”