April Reagan is a box PM for the Visual C++ team and she's one of my favorite people over in building 41. She's got a blog now and in her first post she explains, among other things, what a box PM is. Check out her blog here (rss). Here's a link to her first post, be sure to stop by and say hi!
Introduction: A Day in the Life of a Box PM
A box program manager is basically a project manager without any direct reports (unless, of course you are a box PM lead). In other words, I drive the team from a peer stance. Minding the product cycle and keeping feature teams in check and on schedule is only one small part of the job. Box PMs are ultimately on the hook for everything that ships in the "box." This PM must pick up (or at least prioritize and delegate) all of the loose ends that don't neatly fit into a feature/component area - setup, help, samples, end-to-end product scenarios, customer programs, legal, servicing and external issues, just to name a few.