The Feed view is where you'll find real time updates of any conversations, documents or Tasks that you are involved with. You have your own Feed, in the context, which consolidates everything you are involved with into a single stream.
Groups, Projects and Customers (CRM) also have their own Feeds, so that you can be more focussed on a particular context.The Feed is made up of multiple discussions.
A discussion is a collection of comments, documents and Tasks (items). These are shared with other contexts - Groups, Projects or Customers. In the screenshot below you can see a discussion highlighted within the blue box, which is separate to the other discussions on the Feed.
A discussion can be made up of many different items - comments, documents or Tasks. In the screenshot below you can see that this discussion is made up of a document (1), a Task (2) and some comments (3).
People you share the discussion with have access to all the items within it. In the screenshot below you can see who is shared to the discussion.
For more detailed information on understanding who has permission to view your discussions and items, please view this article