An activity stream is a list of recent activities performed by an individual, typically on a single website. For example, Facebook's News Feed is an activity stream read more...

Object structure

property	type	description
actor	User	Who made the action
object	Object	With what object (Image, Text, File..)
action	String	Which action (Like, Comment, Published, Share...)
target	Object	To which target (User, Event, Group..)

News Feed understanding

Each entity has its own news feed. A user, an event, a group but also entities that you can create for your own needs (dynamic entity). Each post with PUBLIC visibility is also visible from the public news feed.

User create 3 posts:

POST 1 which is published on an Event News Feed with PUBLIC visibility
POST 2 which is published on a Group News Feed with PRIVATE visibility
POST 3 which is published on a dynamic News Feed with PUBLIC visibility

All the 3 posts are visible from the User News Feed. Only POST 1 and POST 3 are publicly visible.