A French aperitif wine. It is a blend of 85% Bordeaux wines (Sauvignon Blanc, Semillon and Muscadelle for the Blanc; Merlot and Cabernet Sauvignon for the Rouge) and 15% macerated liqueurs, mostly citrus liqueurs from the peels of sweet oranges from Spain and Morocco and the peels of bitter green oranges from Haiti. Lillet belongs in a family of aperitif known as tonic wines because of the addition of a liqueur of Chinchona bark from Peru which contains quinine. Lillet is matured in oak casks. While it has been produced since the late 19th century, the current formulation dates from 1986. The formulation was changed only to lower the sugar content; the level of quinine has remained roughly the same.
The final drink needs to be served very cold, because that emphasises the fruit aromas rather than the bitterness. As the drink warms, the bitterness becomes more obvious. You can add ice, but should also add a slice of lemon (orange for Red Lillet) so it doesn't dilute the principal flavours as the ice melts. The finish is dry from the quinine, which is why it is still considered to be fortifying and reviving drink, perfect for stimulating the appetite as an aperitif.
Lillet has recently come back into fashion as one of the components of James Bond's favourite cocktail in Ian Fleming's Casino Royale .007 called it the Vesper (after the female lead character in the book and film) - a mix of gin, vodka and Lillet!