A German variety of mulled wine (see also Glögg, the Skandinavian equivalent). One recipe I've seen includes sugar, cinnamon, cloves, and lemon peel (identical to ccunning's recipe for Glögg, except for the vodka). It's heated up and drunk warm; I first ran into this stuff at the Christkindlesmarkt at Christmastime in Nuremberg. The Christkindlesmarkt is an open-air market thing during Advent in the old center of town, where you can buy tchotchkes, goodies, and hot cups of glüwein to keep you warm. In among the shops there's religious stuff, too. It's serious fun.
It's also sold by the bottle; a week before Christmas, bottles of the stuff were just about everywhere, at four or five marks apiece (more at the duty free shops in the airport, naturally). The exchange rate then was about $0.60/mark. Not bad. I've still got a bottle right here. The label depicts rosy-cheeked happy eighteenth-century Franconians drinking wine in the snow in front of a cheerily-lit tchotchke shop. It says "9,5% vol" on it, too; if that refers to the alcohol content, we can easily imagine why those people are so happy.
I guess you could spell this without the umlaut or diaresis as "Gluehwein", but that doesn't look so appetizing, does it?
The title of this node is spelled with ü, as it should be.