In British English, "sherbet powder" is a fizzy powder sweet which can be eaten on its own or used to make a fizzy drink. You can read about the many varieties of sherbet powders in this Wikipedia article. In the world of Harry Potter, Dumbledore is very fond of "sherbet lemons," although in the U.S. version of the book, the text is changed to "lemon drops."
In the U.S., a sherbet (often pronounced "sherbert") is a frozen dessert like a sorbet, but with a small amount of milkfat. You can find a recipe for strawberry sherbet at the Eat-Drink-Love.com website.