Pooka,
Arrowroot is a starch used to thicken sauces (think fruit pie) and lighten up dense baked goods. It has gained in popularity only recently because of these properties which are useful in grain free diets. It is sold in various sizes. The spice size bottles and larger packages are available, and what you purchase depends on how much you would use this ingredient. It is a good substitute for cornstarch applications since we don't use cornstarch anymore. Many paleo, primal and other low carb recipes call for this. However, keep in mind that it is a high glycemic ingredient (52 carbs per half cup) and should be used sparingly. Whether it is called a flour, powder or spice all refers to its' use, not to different forms of the starch.