From the Mash Blog:
We just started inviting our friends outside Yahoo! to join us in testing Mash: a new approach to online profiles. If you’ve used other online profiles before you’ll feel at home in Mash. But there are some new twists that make things a little interesting and, we think, a lot of fun.
1. You can make starter profiles for your friends. Think: “first round’s on me.”
2. You can leave your profile open to contributions by trusted friends.
3. You can customize your — or your friend’s :) — profile with modules from a growing gallery of apps.
Of course, there are extensive privacy controls in Mash and you set the boundaries that you’re comfortable with.
One last note before you jump in: Mash is still pretty raw — there are bugs and we haven’t gotten to several of the features it really should have.
You notice they talk about profiles and not networking. All interaction seems to take place in each other's profiles. There are no groups or events as in Facebook, no forums as in Ning. See my Mash profile as an example.
UK law librarian Lo-Fi Librarian has put together this screencast (without sound) to give you a tour using her profile as an example. Note in particular the screen that has the privacy settings toward the end (I suggest going to the full screen version rather than viewing here):