Funny you ask, going through the middle of an organization where ProjM is turned into ProdM right now. As far as I can tell very little changes except the titles on business cards ;-)
The distinction I usually make is that a "project manager" is responsible for scheduling: maintaining a picture of the current state of the project, the current best estimates for when things will happen next, and make sure that everything that must happen before the next release is accounted for somewhere. It's not a "management" role in the traditional sense (people management), but it's not an engineering role, either.
A product manager's job is to understand why people are (or are not) buying your product. They need to understand the market, keep track of competing products, and be able to make decisions (or offer advice) about which of a list of potential new features are most likely to make the customer's mouth water. It's fundamentally a marketing role, but, especially with a complicated product, can require a fair amount of technical skill to do.
Disclaimer: This is all Big Company language, but not necessarily identical to Google's language.
Product Manager sounds better. It makes it sound more like you're responsible for shipping something that generates revenue, rather than overseeing an org-chart rework or some other corporate circlejerk.
