Gmail's code suggests that you'll be able to delete the updates you don't like, reply by email or post a comment. It's not very clear what kind of updates you will receive, but they're probably the latest important actions of your contacts from different Google services.
JavaScript code from Gmail
Picasa Web Albums already shows on the homepage a list of recent activities from the people you've subscribed to (not necessarily your contacts):
Google Reader has recently started to display shared items from your Google Talk contacts, a feature that stirred passionate debates.
Google Shared Stuff shows the web pages shared by your Gmail contacts. "A shared stuff page is a collection of all the links that you want to share online. Your shared stuff page is publicly visible, so you can use it you to share links with your friends and others on the web," Google mentions about the obscure service.
orkut, Google's social network, added in September a feature that shows the latest updates from your orkut friends:
The Gmail updates will probably gather all this information, merge it with recent activities from other Google services and display it in a continous stream, similar to Facebook's newsfeed. Deriving a social graph from your address book is not always the best idea if we take into account that Gmail automatically adds people in your list of contacts. Social applications are Google's weakest point, as it didn't manage to create strong communities around any service. Integrating these applications and adding a unified social layer might appear artificial, especially if the users didn't ask for it and they don't have the tools to control the sharing boundaries.