Corrus will give you updates in context to what you are working on. Activity streams will show all activities and actions that your team has performed across cases and tasks.
On the case file, the activity stream is filtered on a case.
On the task interface, you can see the task activity stream which provides updates on all activities that users have performed on a task.
You can upload documents on the activity stream, comment on any action and mention your co-workers. Notifications will be sent to the users if they are mentioned in any comment.