As I mentioned, cassava root starch is very similar to cereal grain starch. It’s made up of the same basic molecules: the very long, straight carbohydrate amylose, and its branched and bushy carb counterpart amylopectin. Thus, it can be made into flour (it’s also make into flakes) and used in many of the same ways we typically use wheat flour.
The one thing that cassava doesn’t have is protein, which means it has no gluten, and so can’t form the stretchy, bubble-holding lattices that wheat flour makes when it’s moistened and worked. That’s not to say it can’t be made into bread, however, especially flat bread, the most well-known of which is Honduran casabe (find out all about the easy-as-pie 19-step process it takes to make it here).
Yet being starch, it’s great at a number of other jobs, especially thickening. Whyfore? Simply because starch molecules, when heated in liquid, tend to disperse into a water molecule-capturing mesh, otherwise known as a gel. If you imagine a granule of starch as a bundle of reeds, only the reeds are amylose molecules, heat causes them to loosen, separate from the bundle and float out into the liquid where they re-bind with one another, except now there are water molecules trapped between them. This is what’s known a gelating, and the gels that starches form have any number of uses.
What does all this have to do with tapioca? Well, you’re just going to have to trust me on that one, and come back a little later.