Like cornstarch tapioca is a “pure starch” which means that compared to wheat flour it has no protein, bran or germ in it and as such packs more of a thickening punch. Tapioca comes in “pearls”, in granules (pieces of pearls) and in flour form. All can be used as thickeners, though the smaller the pieces the more readily they dissolve and the faster they act. Tapioca flour, my preference, dissolves almost instantaneously and because it gels at a lower temperature than cornstarch you can see the results immediately if the mixture is above 140 or so degrees Fahrenheit. Like other starch gels, however it also “un-thickens” when overcooked.
The advantage tapioca flour has over, say cornstarch, is that it’s more effective in acidic environments. Due to its small particle size it also creates clear, glossy gels. One other bonus feature: the gels it makes hold up quite well when they’re frozen. On the down side, those gels don’t stand up well to a lot of stirring, so they’re not as good for sauces. Also if tapioca isn’t incorporated well it can form gummy clumps. Still it’s a valuable thickener to have on hand in the pantry.