They're certainly all of the latter, although I would be greatly surprised to discover that anyone in charge of the material covered there were conservative. After all, it's not just how you approach an issue, it's also the choice of issues you cover, and NPR's net seems a bit broader than other news channels. If that's liberal bias, then...(*sigh*)
I believe I read a study that the station actually gives more air time to "conservative" interviewees than "liberal" ones. I wouldn't say that this translates into absolute lack of liberal bias, but if true I suppose this indicates that the station attempts to incorporate a variety of perspectives.