The abbreviation suggests that it's Pro Gram Manager. As opposed to, say, an Amateur Ounce Manager.

Yes. Exactly. I can't remember what we actually called them at the time (in polite company).

Project Manager - Writes specs and documentation for individual features.

Product Manager - Figures out what new features or changes the product needs to better fit or find a new audience. Usually responsible for doing market research and analysis.

Business Product Owner - Similar to product manager, they own all changes associated with a company product. Position should only exist if product is so large, multiple product managers are needed for it. The product managers would then need to report to or clear their work past this individual.

Program Manager - Everyone above reports to this person. They own all of the products. And possibly oversee development as well.

Just to note, these definitions are no where near universal across organizations.

yeah, not even close. The two or three (only!) companies I've asked tended to have divisions like this, though, just with different names.