Giffard Rhubarbe (rhubarb) liqueur is one of their classic liqueur expressions. It's made with a blend of green and pink rhubarb that they say has a "marmalade-type taste."
Giffard is a French company specializing in a wide variety of liqueurs and eua-de-vie among other products. Some of those extra products are flavored cocktail syrups (or "sirop"). They are generally sweetened with pure cane sugar.