Feed-based page layouts offer a two tabbed page that allows users to switch between a Chatter feed and a detail page for the record. The tabs are marked Feed and Details as shown in the following screenshot:
A prerequisite for feed-based layouts, is the enabling of feed tracking for the object that you wish to create a feed-based layout. Feed tracking allows users to follow records which then lets them see feed updates when those fields are changed.
To enable Feed Tracking, navigate to Setup | Customize | Chatter | Feed Tracking. Here you are presented with a screen containing two panes. The left pane shows the object selection and the right pane shows the field selection as shown in the following screenshot:
To enable feed tracking, select an object in the left pane, enable the Enable Feed Tracking checkbox in the right pane, select the desired fields, and then click the Save button.