It doesn’t contain chocolate’s hallmark ingredient. Some purists (and the Encyclopedia Britannica) don’t even consider it chocolate.
To explain, let’s start on common ground: cocoa beans. When you dry, ferment, roast, and shell a cocoa bean, you wind up with a chocolate nib; grind that nib into a paste, and you get chocolate liquor; separate the liquor into its two components, and you get cocoa solids and cocoa butter. True chocolate contains cocoa solids; white chocolate, however, only uses the cocoa butter.
Beyond cocoa butter, white chocolate contains sugar, milk products, and additional flavorings. Many true chocolates also contain these ingredients, but the cocoa solids are what give them the trademark chocolate taste.