The Forum is organized by Categories > Boards > Topics.
Notifications are an email sent to a User's email address.
Users can set Notifications for a) New Topics in a Board or b) replies to a Topic.
To set a Notification for New Topics, go to that Board (ie Events), then on right click Notify button to toggle on, click Unotify to toggle off.
Similarly, to set a Notification for Replies to a Topic, go to that Topic, then on right click Notify button to toggle on, click Unotify to toggle off.
TIP #1: Users cannot set a Notification for a whole Category or for whole Forum, to see Posts since last visit, click on Show unread posts since last visit below your Display Name in top left, or to see Replies to a Topic you started, below name click on Show new replies to your posts..
TIP #2: To view and modify your Notification settings, from top left menu choose Profile > Modify Profile >Notifications.