confectioner's sugar = powdered sugar + starch. starch keeps it from caking together. (and is also, no doubt, cheaper, so the manufacturer is happy to cut their product with it.)

brown sugar: never buy. see [[https://www.youtube.com/watch?v=LF8cWXlNSEM|this link]] where ragusea explains, and [[kb:culinary:molasses|molasses]]. tl;dr: sugar in general is made by refining molasses out, and brown sugar is made by adding it back.