Birch beer is a variant of root beer traditionally made from birch bark extract and birch syrup and flavored with wintergreen in addition to other herbs. There are many theories as to who invented it and where it came from, but it is definitely American. Many modern birch beers are artificially flavored, like almost everything else in the store, but Journey Food, for example, sells naturally brewed birch beer.
I encountered birch beer my freshman year of college, thinking it was merely root beer by another name, and it took me a few days to realize why I liked it so much. I associate both wintergreen and root beer strongly with childhood vacations in Wisconsin, so birch beer is nostalgia in a bottle for me, but most of the people I know think birch beer is nasty.