Mulled wine is wine, usually red wine, combined with spices and usually served hot. In the old times wine often went bad, but by adding spices and honey it could be made drinkable again. Nowadays it is a traditional drink during winter, and especially around Christmas, to warm you up. Glögg is the Scandinavian form of mulled wine, similar to Glühwein in German.
Glühwein is usually prepared from (not too expensive) red wine, which is heated and spiced with cinnamon sticks, cloves and sugar. (Ubiquitious pre-spiced, bottled Glühwein now has nearly made people forget how easy the real stuff can be prepared, and how much better it tastes than the too sweet, artificially flavoured industrial Glühwein.)
If you add orange juice to it, you get a form of Punsch.