Rice flour is finely milled flour made from white long-grain rice. It is used mostly to make steamed sweet or savoury cakes. Buy rice flour from Asian supermarkets rather than health food shops; the ones from the latter are usually not ground finely enough and won’t result in the right texture.
Glutinous rice flour is finely milled flour made from white glutinous rice. It is used mostly to make sweet dumplings and desserts, giving them a delightfully chewy texture. It is easily available from Asian supermarkets.
These starches are mainly used as thickening agents for sauces, or in a batter for fried dishes. They are close substitutes for one another, but each will give a slightly different result.
By and large, you can get by with just cornflour. It needs to be combined with twice the amount of water to make a slurry that is then stirred into a simmering sauce at the end of cooking to thicken it up. Cornflour also makes for a generally good fried chicken coating – simply dab pieces in the cornflour (no egg or breadcrumbs needed) and then slip them into hot oil.
In terms of thickening abilities, tapioca starch dissolves more quickly. You will also need tapioca starch in combination with rice flour for certain steamed cakes in this book. Potato starch is not used as much, but it does give you even better fried chicken (see har cheong gai, here).
Cornflour is readily found in supermarkets, while tapioca starch and potato starch can be found at Asian supermarkets.