White chocolate is made from cocoa butter, sugar, milk and flavouring such as vanilla. Milk chocolate is made from cocoa liquor, cocoa butter, sugar, milk and flavourings.
Dark chocolate is made from cocoa liquor, cocoa butter, sugar, and flavourings.
Some standards for chocolate do not classify white chocolate as chocolate because it does not include cocoa liquor, only cocoa butter. For example the Codex Alimentarius international standards for chocolate do not include white chocolate, instead they have a separate standard where they are called cocoa butter confectionery.