Found this detailed and interesting answer by Venkatesh Rao, author of Tempo, to the question, What is a day in the life of a Product Manager like? on Quora.
Check email, besides the routine types of email that everybody does, PMs often write a very specific kind of email that I call a “closure, options and actions” email. You are usually managing 2-3 complex conversations at a time, that involve questions about technology, design, what users have said, what management wants, budget issues, HR issues etc. Everybody contributes. Your job isn’t to do all the thinking. But the PM does do one thing others don’t: notice when a conversation is reaching diminishing returns from available information. At this point, you typically step in, frame and summarize what has been said, lay out the options, and either recommend or assign actions.
Get another cup of coffee
Dive into Google Reader or Twitter. Again, mostly you consume news like other people, but there are a few things you not differently that fit well into a SWOT framework (strengths, weaknesses, opportunities, threats). You filter out the irrelevant/personal interest stuff. Anything that is actionable intelligence goes into your processing stream. If it is obvious what to do, you delegate immediately (for instance, I noticed a blog post about upcoming changes in Facebook login protocols a couple of days ago, which affect our in-development FB login feature, so I just forwarded that to our lead devs). If it is not obvious, it goes into your “thinking hopper” (bookmarks, a quick scribbled note to yourself like “think about implications of Google announcement.”). This goes into your Inbox. Most days come up empty, but about once every week, a major or minor item will pop up that needs handling this way. You have no option but to go fishing regularly. I do quick 20-30 minute “market situation awareness” stints every day, and longer “catch up on my Google Reader” sessions of several hours, usually on Monday mornings.
Okay, now for something that finally looks like “real work” to some of you. Roll up your sleeves, fire up your main product management tool. It could be a team to-do list, a formal agile management tool (we use ScrumWorks) or something like Microsoft Project or even an Excel sheet, depending on what aspect of the project you are working on. Your email and paper inbox are probably full of a bunch of pending items for which the right action is to do something in your product management tool. Here, a lot of the grunt work is simply clarifying and defining items for discussion in product management meetings. This means taking an email thread that straggles around the subject “hey, maybe we oughta consider integrating with Twitter” and defining a bunch of meaningful backlog items to be done later by appropriate people. Often you don’t know enough technically to actually define it yourself, in which case you might write the best stub you can for others. Sessions wrangling budgets and resources look and feel similar.
Read the rest of the answer on Quora.