Note to other newbie DIY'ers: the same bulb controls both the tail light AND the brake light. It has 2 filaments, and one or the other may be burned out.
(Maybe this is well-known, but I've not seen this written anywhere (one of the rare times when Google has failed me), and when I had the left brake light burned out, the right tail light burned out and couldn't find the info online, I was THIS CLOSE to taking the van in to my mechanic to have him deal with it. A chance comment by an acquaintance clued me in as to the 2-filament situation. Hope this helps someone!)