White frosting is any white or off-white icing used to coat and decorate cakes and cupcakes. American buttercream (butter and powdered sugar, loosened with milk or cream) is the most common version, though the role can also be filled by Swiss or Italian meringue buttercream, cream cheese frosting, or royal icing -- it isn't simply another name for buttercream. Frosting is generally thicker and more opaque than icing, which tends to be a thinner glaze that's poured or drizzled rather than piped, though the two words are often used interchangeably in casual recipes.
American buttercream is made by creaming butter and powdered sugar until light and fluffy, then loosening it with milk or cream to a spreadable consistency, with vanilla extract as the most common flavoring; it became a fixture of American home baking in the 19th and 20th centuries.
Because its flavor is neutral and sweet, white frosting pairs with virtually any cake flavor, from chocolate to lemon to red velvet. It can be piped, swirled, or smoothed, then tinted with food coloring or topped with sprinkles or edible flowers. Swiss meringue buttercream and Italian meringue buttercream substitute a cooked meringue or hot sugar syrup for some of the powdered sugar, producing a silkier, less sweet, more stable frosting.