Incoming
Your incoming stream is called simply ‘Stream’ and is an aggregated timeline of all the posts of all the users you have assigned to any Circle.
You can also view your incoming stream segmented by Circle very easily with always-exposed controls to the left of it.
Since Google+ is asymmetric, it’s very possible that other users will have you in their Circles even though they aren’t in yours. Any posts from these people will not show up in your Stream but are collected in a special area called ‘Incoming’, listed on the left below your Circles.
In broad Twitter terms, this is the equivalent of there being a special stream of everyone following you, but who you aren’t following back.
Outgoing
All posts on Google+ must be explicitly targeted. You can post to:
Public
- visible on your Google+ profile page
- shown in the Stream of all users who have you in a Circle
- shown in the Incoming area of all users you have in a Circle who don't have you in a Circle
Extended Circles
- shown in the Stream of all users who have you in a Circle
- shown in the Incoming area of all users in the Circles of users in your Circles ***
- shown in the Incoming area of all the users you have in a Circle who don't reciprocally have you in a Circle
Your Circles
- shown in the Stream of all users you have in any of your Circles who also have you in one of their Circles
- shown in the Incoming area of all the users you have in a Circle who don't reciprocally have you in a Circle
A particular Circle
- shown in the Stream of all users in that Circle who also have you in one of their Circles
- shown in the Incoming area of all the users in that Circle who don't reciprocally have you in a Circle
A particular user or users
- shown in the Stream of that user if they have you in one of their Circles
- shown in the Incoming area of that user if they don't reciprocally have you in a Circle
Or... any combination of the above.
Perhaps the most important thing to note is that having users in your Circles doesn't mean you can predict how they are dealing with or if they will even see your posts - that depends most on whether they have reciprocally included you in a Circle or not.
Summary
Facebook, Twitter and Google+ all implement fundamentally different sharing models.
Google+ definitely provides something new but it does so via a model that (perhaps necessarily) is more difficult to fully build a mental model of than the other two services.
Both Twitter and Facebook offer progressive levels of complexity – Facebook Friend Lists and Twitter Lists are both power user-y features that the majority of users don’t need or use (though in Facebook’s case this is doubtless partly because the interface for doing so is both poorly designed and well hidden).
Google+ brings this type of functionality to the core of its service, requiring users to understand something a little more complex before they can even use the service at all. On the upside, it also brings a much slicker user experience to group management along with it.
Whether this added complexity will materially affect take up, or limit it to more technically savvy users is yet to be seen.
* 99% of the time anyway: privacy settings, posting on other people’s “wall”s, excepted.
** I’m ignoring people who protect their tweets - this is a niche use case and not really in the spirit of Twitter anyway.
*** It’s actually more complicated even than that: [removed: link now dead]