What is sweet rice flour?
Just what it sounds like -- a flour ground from a high-starch rice grain. Having said that, it behaves a bit more like a starch (e.g., thickening like corn starch) than a flour. It can be a very helpful part of a gluten-free flour blend
Oh, and it is totally different than "rice flour" (either white or brown) which behaves more like a flour than a starch (if that makes sense). So, you really can't substitute regular rice flour for sweet rice flour. Frequently you will find it in the Asian food aisle, not next to all the other flours in the general baking section.
It is ground from a short-grain white (in other words, husked) rice, and as SeaJambon indicates, it is very high in starch because it derives from a short (fat) grain rather than a long, slender one. The short grained rices are commonly used in sushi, or other "sticky" rice applications, if that helps you.
Look for Mochiko Sweet Rice Flour. It comes in a 1 lb. box, fro Koda Farms. They have a web site, www.kodafarm.com. I have found it in Asian markets and health food stores. Sweet rice is also called glutinous rice, because of the stickyness when the rice is cooked -- as said above, it has no gluten and is very useful for gluten-free uses.