I recently became re-interested in birch beer, having consumed it rather religiously when I was growing up on the east coast. Birch beer is (unsurprisingly) usually made from the bark of birch trees and comes in an assortment of hues, including light and dark brown, red, and purple. Several companies, including Kutztown Soda Works and A.J. Stephans, also sell a "white" (clear) kind made without artificial coloring.
My brand of choice as a child was Pennsylvania Dutch because it sold birch beer in old-fashioned bottles as well as in regular cans. I preferred the former, as they helped me pretend I was drinking real beer rather than just soda pop.
When I moved to Texas, it gradually came to my attention that birch beer doesn't enjoy as widespread a following as root beer, its sweeter, trendier counterpart. I'm hardly prepared to map the popularity of birch beer, but I would hazard to guess that it's far more prevalent in Yankee region of the country given that it's so dang hard to find in these parts. Some Spec's carry one or two varieties, as do a few select grocery stores like Rice Epicurean Market. Boylan's birch beer is decent, as is that made by Sioux City, which is a lovely pinot noir red.
For those accustomed to mainstream super-sweet root soft drinks like root beer and ginger ale, birch beer may initially seem a bit more astringent, even medicinal. It doesn't exactly taste like a tree, but there is something botanical to its flavor. I like to drink it with heavy and/or spicier dishes like jambalaya or jalapeño poppers, and I'm rather picky about the temperature. Too warm and it verges on cough syrup; too cold and it loses its subtle herb notes.