Chartreuse is special for a variety of reasons, but mainly because it has 130 ingredients used in its distilling and aging process. 130! It's also a highly guarded secret recipe known by only 2 of the Carthusian Monks that produce it, at any given time. It has a long history--not entirely unlike Absinth--of government confiscation, clandestine operations, prohibition, and finally acceptance in the early 20th century.
Today, it's made in the area of Voiron, France, near the Carthusian mountains, using the recipes passed down across 4 centuries.
You can sip it, mix it, chill it, heat it, drink it by itself, or with a meal. You can even put it in hot chocolate. It's incredibly versatile, more than any other liqueur I've ever used, but it's also highly flavored, so you don't need to mix much.