View Discussions through SharePoint 2010
When first navigating to a discussion board in SharePoint 2010 (for example, say you clicked a link on the Quick Launch menu and navigated to the Team Discussion list), you first see the Subject view of a discussion board. The following list helps you understand the different view options:
The Subject view shows a list of subjects (also called topics or discussions) with the creator of the subject, number of replies, and last updated columns.
The Flat view is visible when you click a Subject link. In this view, you can see all discussions posted in chronological order. This view also has a Reply button for replying to the original subject post or any other reply in the thread.
The Threaded view is an option from the View drop-down list in the Current View section of the List tab. When users switch to this view, they see posts and replies in the hierarchy they were created. In the Flat view, all posts are shown as the same level, but in a Threaded view, replies to replies are shown grouped and indented.