Chartreuse is an aromatic liqueur, originally made by French Carthusian monks. It is made from a mixture of 130 different herbs, and is aged on oak barrels. Chartreuse come in two varieties, green and yellow.
Green Chartreuse gets its green color from the addition of chlorophyll. It is higher proof, drier and more spicy than the yellow variant.
Yellow Chartreuse is made with the addition of honey, and is colored pale yellow with saffron. It is lighter and sweeter in flavor and is lower proof than the green variant.
Classic cocktails usually call for yellow Chartreuse, while the green version is more popular nowadays.