You're also limited to what you can do. Path allows you to share and view photos, that's it. There are no likes, retweets, comments, walls, or anything.
Path was started by former Facebook executive Dave Morin. Right now, it's only available as an iPhone app. Those without iPhones are stuck, although if you are friended by a user, you can view his or her photos on the Web.
More differences from general social networking behavior: no lurking. Path has a feature it calls "Seeing." You will be able to tell which of your friends have seen any particular photo. Path says, "The idea is that understanding enables trust and storytelling amongst close friends and family."
As pointed out by Scobleizer, there's another difference: you can tell when someone drops you from their Path list. Based on how your Path list is supposed to encompass your true friends, this might be by design. Still, how would you like to get a "Dear John" letter from Path?
At any rate, there's one thing for sure: you won't need to run a National Unfriend Day (which, by the way, is Wednesday) for Path.
Try out Path if you have an iPhone and let us know what you think. We have Android smartphones, ourselves (though we have had iPhones in the past).